Ok, here's the problem - got a '97 XJ (4.0, auto, 106,000 mi) with a momentary throttle hesitation that only happens about 15-20 seconds after starting it. Doesn't matter if it's cold or had been running for hours - if I shut it off it then restart it after a few minutes, 15-20 seconds after I start it, I get a hesitation from the engine like it's starved for fuel. This occurs for 2 or 3 seconds, then it takes off and runs normally. It won't happen again no matter how long I drive it, unless I shut it off again. I can drive it around town all day idling in traffic, floor it to merge onto the freeway, drive for hours at 80 mph - no problems. But if I shut it off - even for just a few minutes - and then restart it, 15-20 seconds later, I get this momentary hesitation that feels like the fuel pressure drops off suddenly, then comes back to normal and stays there. I first noticed the problem several weeks ago, but it didn't happen all the time at first - but it's starting to get worse quickly.

Entire fuel module (pump, regulator, filter) is new, replaced just a few months ago (original pump went out). Injectors are new, replaced a few months ago when I swapped in a '99 intake manifold. Coil is new MSD, cap & rotor are new, wires are ACCEL, pretty new, and in great shape, plugs are Champion truck plugs gapped at .045" (this may be too much, I thought maybe the MSD coil would handle it). Both 02 sensors are only a few years old, entire exhaust is new, with Pacesetter header, mandrel bent tubes, Random Technologies cat, and Flowmaster muffler installed at the same time I swapped the intake manifold.

I never get a Check Engine light and the key on-off 3 times trick shows no logged trouble codes, so that leads me to believe it's not a sensor going bad. With an entire new fuel system, I don't think it's a fuel pressure problem, plus the fact that it only happens right after startup, then always runs great after that. I never have to crank it very long to start it, usually just a second or two and it fires right up, cold or hot.

So, anybody encountered a problem like this? Is it a sensor (TPS, CPS, CTS, IAT, MAP) going out that's freaking the ECU? If so, why no codes?

same exact thing happens to me, but the only difference is it not a time issue it's a load issue. I can crank it & let idle for 5 minutes no prob, but the first time I actually get in and drive (put a load on it) it will hesitate exactly as you explained above. Most likely when you crank it you immediately get in & drive it so I'm positive it's the same thing.

I'm as stumped as you...

Same thing on my 97 TJ auto, though seems more intermittant.- not every startup, maybe few times a week. You mention a fuel pump change and that makes me wonder if it could be related. I got a bad load of fuel (gals of water in a fill up) and managed to nurse it back to my shop over 100mi away with a couple gals of alcohol to help with the water. Thing would run for short time and then filter would apparently clog and barely run, had to switch off ign (shutting down fuel pump) for few seconds which released suction at pump and would then restart and run for a little longer - took 6hrs on a 2 lane with traffic.. Point is that fuel starvation felt very similar to that hesitation and now I wonder if that filter screen might slightly clog during initial load being put on engine and then clear itself.. Might not get a code set if low fuel pressure for only very short time and that filter sock is designed to not pass water, it just sort of forms a gell over it. Maybe a little water in tank. Condensation?

I don't think it's water in the gas or anything blocking the pickup. I've run drygas and concentrated fuel injector cleaner through there on nearly empty tanks with no difference, plus the fact that I never have problems at any other time other than just after starting it up. I drove nearly 300 miles today on service calls, both in town and several hours on the Interstate at 75-80 mph with no problems. I did experience the "hesitation" just after starting it up 2 separate times, though. Sputtered for a few seconds, then cleared out and ran strong for hours after that.

It's almost as if there's a glitch in the ECU programming that causes a problem when it switches over from open to closed loop operation. If it was a true fuel delivery problem, I'd think it would give me problems at random times, or under severe load conditions (WOT up a long hill)..but it doesn't. If it was a sensor going out, I'd think the ECU would detect the voltage from it as being out of range every now and then, and log a trouble code...but it doesn't. If it was a wiring problem like a loose connector or momentary short, I'd think the problem would come and go at random times, but be more noticable on a bumpy road or over a speed bump or railroad tracks...but it doesn't. The problem only occurs a few seconds after starting the engine, then clears itself and never occurs again until the engine is shut off. After it occurs the truck runs fine, no matter how hard or how long I drive it.

WEIRD.

I'm going to try a few things this weekend (regap the plugs to factory specs, disconnect the battery for a few hours to clear any "hidden" codes, reseat all the wiring connectors I can get to under the hood while the battery is disconnected, use an ohmeter to test the TPS, look for cracked vacuum lines, etc.) If I find anything I'll let you guys know.

Check the wires going to the crank sensor. SInce you recently had the intake and exhust off they could have gotten burned through on the exhaust. This will cause an intermittent problem sometimes that will give you a start and die condition. It could just be that yours hasnt gotten shorted the wires out completely yet.

Paul

earlybroncoguy, did you are anyone ever figure out this problem? I'm having exact same problem as you. Not under a ld, but as you start it. I could be running it all day and shut it down on a trail and there it goes. Cuts and sputters and if I give some gas, cuts out slightly and then vroom off we go never to do it again still I shut it off and start it back up. Mine is on 1996 XJ 4.0 that's now in a 95 YJ.

I have changed everything but the fuel pump and it's in the parts getter to try that but I wanted to narrow it all down.

Did you ever find out anything else on this?

Thanks for any inside to this Grimlin in my Jeep.

Maybe your fuel pressure is bleeding off somewhere when you shut the engine down, and when fired back up you are getting an air pocket in the fuel system. Get a gauge and shut the engine off while some one is monitoring the pressure gauge. I have no ideas about were the fuel would be going unless you have an injector with some trash in it or something like that. It maybe going back to the tank? Just a thought. :confused:

Maybe your fuel pressure is bleeding off somewhere when you shut the engine down, and when fired back up you are getting an air pocket in the fuel system. Get a gauge and shut the engine off while some one is monitoring the pressure gauge. I have no ideas about were the fuel would be going unless you have an injector with some trash in it or something like that. It maybe going back to the tank? Just a thought. :confused:

I kinda thought that too, but I have sat there after I started reved it up and then take off fine......the putt, putt, putt, cough....vrooom. Everytime. Then runs fine.

I have turned it on, the off, then on thinking it might hold the pressure in the line. Didn't seem to help either.

It's more of annoying than anything. Only time it really sucks is wheeling b/c I can't turn it off if I'm watching others wheel up stuff waiting my turn b/c if I do it will do it on the trail. Kinda like a bucking horse in 4LO. :mad3:

This is just a theory but you start up and ride without any problems. You turn off the hot engine. Crank it back up after a few minutes and it starts fine. Idles OK, until you give it gas, then it studders for a few seconds and then takes off.

Vapor Lock. Think about it. When you shut down a hot engine, the fuel line and/or fuel rail gets hot enough that the gas boils or vaporizes. When you start the engine back up, the fuel pump provides enough pressure and just enough fuel for the engine to idle. The vaporized fuel does not get plurged completely from the fuel rail until you get on it and increase the fuel flow through the injectors and force the vapor out of the fuel rail, hense the studder.

Just my experience.

Scott

I was thinking vapor lock too this weekend when mine was stalling quite a bit. My fuel cell was pretty warm due to the warm gas being returned. Sometimes when I restarted the engine it was smelling very rich in the exhaust.
Anything to do about that vapor lock/rich running problem?

This is just a theory but you start up and ride without any problems. You turn off the hot engine. Crank it back up after a few minutes and it starts fine. Idles OK, until you give it gas, then it studders for a few seconds and then takes off.

Vapor Lock. Think about it. When you shut down a hot engine, the fuel line and/or fuel rail gets hot enough that the gas boils or vaporizes. When you start the engine back up, the fuel pump provides enough pressure and just enough fuel for the engine to idle. The vaporized fuel does not get plurged completely from the fuel rail until you get on it and increase the fuel flow through the injectors and force the vapor out of the fuel rail, hense the studder.

Just my experience.

Scott

That does make sense, but I had been told fuel injected motors did't get vapor lock b/c the fuel presure was contantly suppose to be on it. I need to run some more test on fuel pressure, I still haven't ruled out fuel pump, but that doesn't make sense to me either if it works after it sputters. Just driving me nuts! :mad3:

Fuel injected motors can have vapor lock. On Porsche 914, the original fuel pump location was in the engine bay and that's where the lock took place, in the pump. So I figure it can also happen in the rail.

The key point with my observation is that when the engine is hot and not running. The fuel is not being circulated so it gets so hot it boils. On the OBD-I engines the fuel regulator at the fuel rail allows the build up of the pressure to bleed off. On the 96-up engines I can only assume the regulator in the tank will allow excess pressure to bleed back as well. The end result is that when you start a hot engine, the gas/vapors that are in the fuel line enter the fuel rail and the vapors are plurged through the injectors (hense the hesitation in starting) until the liquid gas displaces the vapors. There is still vapors trapped in the fuel rail until the fuel flow increases and thereby plurges the remaining vapors.

Does that sound pluasable?

Scott

Ok, let me add this and see if this throws everyone off. Earlybroncoguy's Jeep does it when it's hot, mine does it everytime I turn it off weather it's hot or cold. Now let me add this to it, b/c I just thought of it when Scott mentioned pre 95 and after 96 obdI/II.

Mine had a 95 2.5L in it I put in a 96 4.0L out of an XJ into it. Just like Scott said there was a difference in the location of the regulator and a change in the return line. The regulator on the XJ was in the tank mine was on the rail. So I took the top half of the YJ sending unit and put the XJ pump and regulator on it and dropped it in my tank. I routed the fuel then to pour back into itself and there is no true return line from the manifold.

I was reading some searched articles where some guys had ran 2 pumps but it usually was on a 350. What puzzles me is it's the same pump that ran it in the XJ, just in a YJ.

Did I loose everyone yet?

Sand, forget the two pump idea. Check you fuel pressure at the fuel rail when it's running, after you turn the engine off, and five minutes after shut down. You should not see that much pressure bleed off maybe 15-20lbs(which is close to half but you'll loose pressure just checking it with the gauge). Then watch the pressure while you restart the engine and for the first minute. See if it stays around 46-52 psi. I'd start with that. Obviously your issue is not solely caused by vapor lock, since it happens when the engine is cold as well. What ever is causing the main problem (pressure problem?) may be a contributing factor in the occurance of vapor lock.

Mine is doing it only right after starting, or it was until this weekend when I got into the stalling and smelly exhaust.

I'm running a 98 TJ 4.0 with a rail from a 1994 XJ 4.0 with the pressure regulator and return line. Don't know what the pump stock application is. After a long down hill ride, it was hesitating and stalling.

Very curious that it's happening with a new stock setup though for earlybroncoguy.
We have another problem I think.
I have a tentative appointment with a friend of mine who runs a shop.
If I can't figure out the problem he's gonna run a bunch of computer tests and stuff.
I'll let you all know what he finds if it does indeed go that far.

I have the same problem on my 98 cherokee as the original poster. It is kind of like a momentary power loss. From what I understand there was a TSB on that a long time ago where the third injector loaded up (vapor lock?) because of the built up pressure and heat and to remedy this jeep put in a heat shield between the manifold and the third injector I believe. I also know of one person that changed the fuel pump in the tank and it helped his trememdously. --------------Kyle

Sand, forget the two pump idea. Check you fuel pressure at the fuel rail when it's running, after you turn the engine off, and five minutes after shut down. You should not see that much pressure bleed off maybe 15-20lbs(which is close to half but you'll loose pressure just checking it with the gauge). Then watch the pressure while you restart the engine and for the first minute. See if it stays around 46-52 psi. I'd start with that. Obviously your issue is not solely caused by vapor lock, since it happens when the engine is cold as well. What ever is causing the main problem (pressure problem?) may be a contributing factor in the occurance of vapor lock.


Will do, thx everyone for ideas. :D

I hope you all keep digging. I have the same issues with my 98 4.0, only when warm, usually during a short stop and restart like on the trail. Happens very infrequently, but enough to get my attention. I was originally thinking TPS, but vapor issues makes sense.

I would guess a few of you already replaced the TPS with no change??

I've replaced the MAP & TPS w/ no luck....

On all stock locations and most swaps the fuel lines from the tank connect to steel lines near the rear of the engine on the driver's side, then run along the BOTTOM of the intake manifold before connecting to the fuel rail. Look at the proximity of the steel lines to the exhaust manifold. I never did like that design. I have not done it but you could get some thermo tape and rap the fuel lines in it as well as the fuel rail. If you really want to see if this is the issue, take the hood off one afternoon and see if your probem still presists.

Good idea about removing the hood Scott.
Anyone here actually have time this week to give this a try?
Just looked at my new lines and I followed the path of the stock steel lines. Even tied the new ones to the old one. Maybe I'll need to move them.

While I don't have the hesitation that you guys have, I have used some DEI brand insulating products on my 4.0L conversion. If you look at the fuel rail and lines you can see where I wrapped them. There is a adheasive backed sheet that I cut and placed on the bottom of the intake manifold to help deflect the "header" heat. All items were purchased from Summit Racing.

http://www.preciscom.net/gmoabrim/afilter-01.JPG

GM

Reading through the post, I think it sounds like a sensor/ECU issue... you may not be throwing codes yet, but it sounds like something's getting a voltage spike after start-up (IAC, TPS, etc) and causing it to bog...

Google for a diagnostic/trouble shooting table, it'll tell you how to chase your sensors with a voltage meter and you can verify what readings you're getting at the sensors - at the very least, this will help you eliminate items from the possible culprit list.

Good luck!

Ok I have 2 things I learned....

First how I fixed mine and what was wrong with it. Rember how I talked about how mine was set up with the regulator in the tank and a return line in the tank? Well my fuel line had split open. I guess fuel line does not like to be submerged in fuel all the time. Replaced it with fuel injection line, gave the throttle body a scrubing and whooo hooo back from the dead.

BUUUTTTT I might have some insite into some of the other problems with 4.0's listed above if you have a YJ/TJ. I was talking to a buddy of mine who has had the same type problem that everyone is discribing and what his problem was, was his fuel tank had slightly shrunk. I guess from combination of being smacked on the bottom, maybe heat from the exhaust but anyway the return line (metal one in the tank) was touching the bottom and restricting his fuel flow.

I checked mine just for shits and giggles and it was definatly touching the bottom of the tank and if I was using it as a return line it would not be working. I cut mine down about 3/4" just to do it. Never know when I might switch things around again and might need it. Doing it then would just same me the time of dropping the tank again.

good try but the 'shrinking tank' theory doesn't have legs b/c I have the stock sender in an aluminium cell and have had the problem before & after. Thanks for the try though.......we'll eventually figure this out if people keep throwing the ideas out there.... :D

Greetings. I have the same symptoms on my 96 ZJ so I am following this closely. My experience:

Symptoms started early when rig had very low mileage and was stock. Back then it happened only once in a great while and was just a curiosity. Now mileage is 100K and the hesitation thing is constant. The mechanic that does a lot of the work on my ZJ talked to the dealer and they implied it was a fuel/heat thing and gave him a handful of foam insulators to cable tie to the injectors. That didn't do anything to fix the problem.

It sure does seem like a heat induced fuel delivery issue but the problem I have with that theory is why (at least in my case) does it get worse over time? Seems to me this would be a constant from day one.

Regardless, I like the idea of insulating the fuel rail and I may try that unless someone comes up with a better solution.

I know my problem has nothing to do w/ heat as it only occurs at start-up & only happens once until again at the next start-up. it'll happen the first time I put load on the motor after sitting overnight at 30* outside OR after cranking it after stopping for gas on a 90* day w/ the motor temp @ 210*....no difference.

it has to be some sort of fuel delivery sensor that takes a load to charge up or something....

have you guys thought about 02 sensors? i had generally the same prob on my tj(no check engine light on) and a buddy of mine who owns an alignment shop told me to check and replace my o2 sensors....it worked

mine does the same thing, 98 xj, but my engine light is on. i'll be changing my o2's and i'll let you know what happens.

O2 sensor sounds like that could be it - they need to be heated up to work properly, either by exhaust heat or a built in heater (4 wire type). That would explain why it ocurs just after start up but not after you get thru that first hesitation. Don't have my manual handy but does anyone know whether TJ uses a 2 wire or 4 wire type?

i meant to elaborate....at first the check engine light wasnt on but after a month it came on as the hesitation got more persistent. ever if your check engine light isnt on i would still try replacing them. if you dont feel like buying both of them (like 100-120 bux), try the one on the header first...that one is usually the culprit....


my 97 tj is a 4 wire. i can get the part numbers for you tomorrow if you would like

I know my problem has nothing to do w/ heat as it only occurs at start-up & only happens once until again at the next start-up. it'll happen the first time I put load on the motor after sitting overnight at 30* outside OR after cranking it after stopping for gas on a 90* day w/ the motor temp @ 210*....no difference.

it has to be some sort of fuel delivery sensor that takes a load to charge up or something....


your problem sounds like a vacuum leak. make sure that all those shitty little vacuum hoses running everywhere are connected and dont have any leaks.

my buddy had the same problem with vacuum leaks in his ranger 4.0 (which are notorious for vacuum leaks around the head/intake)

try putting it on a scanner or pressure test the heads. that what we did on his, pinpointed the leak to the intake/head junction, tore it down, put new seals in everything and his problem went away.

P&T Jeeps; We most likely still have the same problem. Stone cold mine will not cough and puke on first acceleration, but it will be a little weak during that same time. Heat just makes it dramatically worse.

O2 sensors sounds like the logical culprit but I have had this issue even with my original sensors that were replaced maybe 18 months ago. I know there is 12V going to the heaters because that is what shorted to ground internally one day on the highway. That was fun.

I did order from Summit some of the metalized convoluted tubing to insulate the fuel rail and such. I'll pull the heap out later this week and try play with it. May be just treating a symptom but if it makes the situation tolerable for $25, I'm in.

O2 sensor sounds like that could be it - they need to be heated up to work properly, either by exhaust heat or a built in heater (4 wire type). That would explain why it ocurs just after start up but not after you get thru that first hesitation. Don't have my manual handy but does anyone know whether TJ uses a 2 wire or 4 wire type?

Our 92 Cherokee did the same thing. When we took it in to have codes read, there were none. I brought up the O2 sensor to the mechanic and he said that it would throw a code, but I'd bet it was the O2. We had a 3inch exhaust and no cat, with the exhaust being so modified.....sounds logical to me.

The O2 sensor is not being used during startup or warm up. During those times, a pre-determined fuel program is used. According to FSM and techs that I have talked with, the O2 sensor is used during crusing only.

good try but the 'shrinking tank' theory doesn't have legs b/c I have the stock sender in an aluminium cell and have had the problem before & after. Thanks for the try though.......we'll eventually figure this out if people keep throwing the ideas out there.... :D

Yeah, wouldn't say that would be an issue with a cell but if I had mine in the stock configuration it would have been and issue with mine, like it was with my buddies. I think mine is more from the skid plate being banged up and pushing up rather than it really "shrinking".

I think it's worth a look especally if you have a banged up skid plate and stock tank. :)

also...check the fuel pickup inlet in the tank....sometimes the foam deteriorates and gets caught in the pickup of the fuel pump.

also...check the fuel pickup inlet in the tank....sometimes the foam deteriorates and gets caught in the pickup of the fuel pump.
there is no foam in a stock tank.


I'm starting to buy into the vacuum theory.....it seems to make sense.

im not talking about foam actually in the tank....on the bottom of the fuel pump inside the tank there is a plastic strainer(about 2''x4'') that is lined on the inside with foam. i dont think that is your problem because that will tend to bog down/cutout at ALL times when a load is put on the engine...

but check for vacuum leaks...if one of those little 16 gague vacuum hoses are disconnected somewhere it can cause the problem you have stated...

I have the same problem with my 97 2.5. When mine starts acting like this I get a can of Cat coverter cleaner and add it to the gas. Once it's through the hesitation is gone for several thousand miles. Don't know if its actually the cat. clogging up or if it's fuel system related but it works for me.

Sorry it took me so long to get back to y'all, busy at work.

Mine's fixed. here's the problem on mine, though I don't think it will help most of you.

I'm running the summit cell in the bed with an inline pump and a 94 rail with return. My computer was confused because I didn't hook up the sender to the guage. Apparently, the computer thought it was always empty and therefore put too much fuel through to get it running, hense, the hesitation, occasional stalling and rich exhaust.
I hooked up the wires to the GM sender in the RCI cell to get the variable voltage. Now as fuel burns, the guage fills up. Can I make millions off this system that creates fuel?

Also changed the plugs, wires and cap. Problem solved, no more codes, runs great.
good luck.

Same thing here, whit the only difference mine is a 4 cyl, i got it fixed replacing the fuel filter and the oxigen sensor, i dont know if that help but work for me