TBI - stumped...

[woody]

1991 350 Chev TBI - idles great, "casual" accelleration is great, mash the pedal and it bogs down. Back off the pedal, and it accellerates casually...mash again, and it bogs.

Ran WinALDL, pulled a code 44, replaced the O2 sensor. no change.
Replace the TBI basegasket, no change.
Air cleaner is new.
Fuel pump is new.
Fuel filters are new.

NO other codes....drove it last night about 10pm (my neighbors love me), WinALDL shows nothing. Did not turn on the DataLogger, but can if someone thinks that info will note something. All sensors appear to be sending information in correctly.

I want to mash

[Big DW]

I have the identical problem....my fuel filter was bad so I replaced it...that improved things but still bogged when mashed....I finally went to carb and HEI for the rest of the season....I was pulling code 43....my Certified GM buddy thinks it's a bad ECM....this winter I'll be installing a Howell TBI harness and finding a different ECM (7747)....you may want to check the filters one more time....mine was new and it was still plugged....good luck.....

Hope you can mash soon.....

[FredTheMonster]

may be a stupid question but... thought I would speak up to make sure -- is your fuel pump strong enough?

My RamJet needs a TON of fuel, pressure filled lines (45psi min.) and a return line... not sure of the pressure requirements for a TBI...

Hope it helps,
RamJet 350 MASHES GREAT...

[woody]

Originally posted by FredTheMonster
may be a stupid question but... thought I would speak up to make sure -- is your fuel pump strong enough?

yeppers...E8153 pump is the recommended external for the system...and it initially worked great (Feb until July), then had issues during BHCC and ever since. The common thot at BHCC was a fuel pump problem, but the new one installed last night changed nothing.

Swaying between TPS and MAP right now....probably need to read the datalogger tonite and see if I can spot something amiss, or hope that some added road time kicks up a code...

[yoda]

Hey Woody,
Has it run good before and this is a new problem, or is it a new TBI setup and this problem is there?

Base timing set with EST unplugged?
You running EGR? (open at wrong time?)
Have a knock sensor? (ping causes timing retard)
GM prom or custom burn?
Good fuel pressure/volume?
Fuel filter on the suction side (bad) or pressure side (good) of the fuel pump?


Note: I see you posted some of the answers while I was typing.

[woody]

Originally posted by yoda
Hey Woody,
Has it run good before and this is a new problem, or is it a new TBI setup and this problem is there?

Base timing set with EST unplugged?
You running EGR? (open at wrong time?)
Have a knock sensor? (ping causes timing retard)
GM prom or custom burn?
Good fuel pressure/volume?

This was a motor pulled complete from a running 350 van. Only changes were relocating the knock sensor and getting a stick-shift prom burned for it. Wiring is stock, but cleaned up. Timing has never been touched from stock. In reading the WinALDL live data on the laptop last night, all the sensors are putting out properly.

It appears to be a load-related issue. Rev it up in the garage, and it does fine. Put in gear to drive, and it bogs down hard.

[yoda]

While U were typing I also asked if filter is before or after fuel pump

[woody]

filter is on the suction side....standard old clear filter tho, and "shouldn't" cause any issues IMO....from "all I've researched" you should run a filter before the pump to ensure any debris from the tank doesn't wreck the pump....

Plus, I've "always" run the pre-filter, and the problem is new.

[TLCObsession]

Woody -

ARe you running an HEI? (Assume so...) Anyway what kind of advance do you have? If mechanical, maker sure everything is clean (rust-free) and smooth - no grease, light oil. You can change the weights and springs. If Vacumn is your diagram intact?

I had an HEI that would not pick up acceleration properly (not on an HEI) and I chesed fuel problems and finally figuredout there was too much white lithium grease under the weights.

Jim

[Ballard]

I'm liking the knock sensor idea. If it started in BHCC and continues, maybe it's something loose in or around your stock fuel pump area. (where you have the knock sensor mounted) Can you run WINALDL while driving and check timing advance? Pull the distributer cap and rotate the crank by hand and see how much timing chain slop you have. Lots will cause problems in itself, but might also contribute to the knock sensor picking up stray noises.

[woody]

ARe you running an HEI? (Assume so...) Anyway what kind of advance do you have? If mechanical, maker sure everything is clean (rust-free) and smooth - no grease, light oil. You can change the weights and springs.

No, it's not HEI, and no vacuum advance. Dist has not been moved/touched since install. Might be worth a check tonite tho, something may be loose around there....however, since it's a load-only problem, I'm not sold on it being electrical but vacuum related...

I'm liking the knock sensor idea. If it started in BHCC and continues, maybe it's something loose in or around your stock fuel pump area. (where you have the knock sensor mounted) Can you run WINALDL while driving and check timing advance? Pull the distributer cap and rotate the crank by hand and see how much timing chain slop you have. Lots will cause problems in itself, but might also contribute to the knock sensor picking up stray noises.

BHCC was terrible....but the symptoms were starting prior to that event. Even the first day, enroute to the trailhead, it wasn't accellerating smoothly...gradual accelleration was fine, but push hard on the hills and it just wouldn't pick up speed. I'll check the knock mounting tho tonight. Also, I would expect knocks to be most prevelant when there is no drivetrain load on the motor, and it doens't have issues when NOT under load.

Thanks for the thoughts gang...keep 'em coming!

[Rascal]

Woody,

I had the same symptoms with my 92 350 TBI that came OUT OF A VAN - DOWN BY THE RIVER. (In my best Chris farley imitation).

I replaced the TPS (Throttle Position Sensor) and now it runs like a scalded cat.

Bruce

[cruiserbrett]

Sounds like an ECM, I would guess. It could be running on the netres(the "limp home" mode resistor pack that is part of the memcal) cause they run, but like shit and dont like any kind of rapid changes...

Is there anychance the computer got really hot at BHCC? this can wreak havoc on them... Also, is the little window in the PROM chip covered? I have seen a few that werent and ambient sunlight can eventually erase them...

[helocat]

Brian,

I keep thinking fuel delivery problem. I run a small oil filled fuel pressure gage under the hood. It is plumed right in the primary fuel line just off the manifold. (Mounted off the fender) This has proven it’s self invaluable with trouble shooting issues. If you don’t have one I strongly suggest installing one, $19.00 from Jegs, so cost is not a major issue. (Unless you run AN lines like I do, then your looking at about $30 more in fittings)

Once you have the gauge in place you can quickly rule out ANY fuel delivery issues just by opening the hood. (Great for on the trail! Or in your case on your side on the trail ) The new pump or new filter may have issues right out of the box that you recently put in.

1. I am still thinking delivery fuel problems.
2. I would try another "Known heathy" ECM.

Mark

[Mr McGee]

i am having the same problem with my uncles truck i borrowed. changed cap, rotor, fuel filter, o2 sensor and still no change. when you figure it out, let us know! btw, my bet is also on the TPS.

[Coreyperez13]

Just a couple other ideas.

What type of exhaust are you running?

My dad had a truck that did the same thing (actually it was a .... VAN G1500) and he found out that the distributor gear had spun, the roll pin had failed.

Corey

[woody]

Is there any chance the computer got really hot at BHCC? this can wreak havoc on them... Also, is the little window in the PROM chip covered? I have seen a few that werent and ambient sunlight can eventually erase them...

Computer is mounted under the glovebox, out of direct sunlight, and with airflow all around it. Air temps were 100+ at BHCC, but the issue even occurred that morning prior to the daily heatup...and the chip is covered by the stock cover, with seal, so don't see how any light could enter that...it runs/idles great, and will accellerate fine...just have to gently ease the throttle down, not "mash" it...

I don't recall the problem occurring prior to BHCC...it was not a problem during our trips to Slade/Tellico in June or our trip to Attica in mid-June.

I keep thinking fuel delivery problem....The new pump or new filter may have issues right out of the box that you recently put in.

I'll check fuel pressure tonight, easy enuf to do. Would prefer not to plump in a gauge, just one more thing to break when the truck is on it's side... lol

What type of exhaust are you running?

hugger headers, O2 sensor in the DS prior to the collector, custom crossover below the pan, single 3" out the back. Stock van had no cat and no air pump, and ran a similar setup. O2 is "as close" to stock length from the ports as possible.

TPS is a cheap/easy fix. Hopefully, the datalogger will note some "odd" numbers tonight and I can track it to something specific...just hate to start changing things since the sensors get pricey quick!

Thanks for all the comments and ideas!!

[reorx]

TPS: Basically its probably not giving you enough fuel boost and leaning it out real bad, which will then cause the knock sensor to hear the pinging and then retard the hell outta the timing. Thus you get nothing...

The datalogger should show you the TPS percentages as you depress the accellerator. I initially had mine hooked up backwards and it backfired every time I hit the gas...

Let us know if you find the culprit!

[woody]

Last nights work:

replaced the cap/rotor
reset the timing
adjusted the idle speed
grounded the park/neutral wire
tested the TPS...all ranges normal (5 volt constant, ground solid, variable seemed very smooth thru the range)
took it for a spin with the laptop recording everything, got a code (don't recall number) noting a bad MAP Sensor

My guess all along has been a vacuum issue, so it was nice to see that confirmed.

Will be picking one up at lunch today, installing it while the truck is on the trailer tonight, then leaving for a weekend of wheeling...hope this solves it!

thanks for all the input gang!

[texican]

Originally posted by woody
Last nights work:

(don't recall number) noting a bad MAP Sensor

thanks for all the input gang!

I had a spare MAP at BHCC you could have tried

Butch

[woody]

Originally posted by texican
I had a spare MAP at BHCC you could have tried

DAMN...all you told me about was the spare TPS!!!

day late, $30 short....lol

[texican]

Originally posted by woody


DAMN...all you told me about was the spare TPS!!!

day late, $30 short....lol

I had spares of just about every component on the TBI system.....

which we swapped most of them out during that cluster F*** trying to fix my junk.....
Butch

[wayneraintree]

hello,

Was this issue ever resolved? I am having similiar issues, been searching forums for long times learning and trying different things. My TBI 350 gives me a code at idle, runs really rich too, then the code dissapears when I raise the RPM's over 1000. When I come to a stop, the idle is at 1000, after 15-20 secs it kicks down and the "soft-code" turns my friendly little green SES light on, like I said, it then turns off when I am driving. Runs really rich. I had to adjust the "throttle" screw (so I thought, not sure now) when I dropped the V8 in my 55 to make it runs at all as the mechanism was not touching the "at rest position". I did this to get it at 500 RPMS for idle, then put the ALDL in diagnostic mode, disconnecting the IAC, set the rough idle to 450RPM's, shut it down, disconnected the A/B, reconnected the IAC, then drove for 20 minutes..no codes....then.. code appeared when I came to a stop like before. Stumped me .... new TPS, EGR works mechanically at least, IAC reads proper OHMS, new O2 sensor, knock sensor, MAP sensor (got code for this )... uh what else, I get a code reading "rich exahust" .. sent me to the EGR and it is OK, like I said, been trying some things.. anyhelp would be appreciated. 93 TBI 350 5.7 stock harness weeded(from 2wd K1500, wish I knew more), SM465, 7447 ECM...

[orangefj45]

i've found a couple of very common problems on the tbi's.

1) oxygen sensor too far from exhaust port. this sensor should never be mounted more than 16" from the head to header/manifold surface.

2) fuel pump and filter. i've had very good luck with the walbro pump sold by howell. needs to be 18psi, not 10, not 35, not 45. too low a pressure and the injectors won't fire properly. too high, and you're gonna blow up the lines and possibly diaphram in the throttle body itself.

the knock sensor is not essential, and can be mounted just about anywhere on the vehicle. it does NOT have to be mounted in the stock location. a lot of competition rigs run them on a bung on the frame itself to avoid knock sensor to exhaust clearance issues. and that works just fine.

[sprung over]

I have run into this quite a bit on different vehicles. I went through two new fuel pumps from Downey on the first one. Plenty of pressure but no volume. Since then, I have found numerous fuel pumps that were not supplying proper volume. Airtex makes a good TBI replacement pump.
Also try running with your fuel cap loose or off.