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Old 09-01-2011, 07:10 AM   #1 (permalink)
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tbi rough idle when warm

OK so Iv'e had this problem and have been searching all over the net for the last three days for an answer to no avail.

here's what I have
sbc
comp cam off road cam
zz4 intake
stock tbi
18 psi fuel pump

Iv'e been working on this retro fit for the last two months and have never had much of an idle problem, at least not after i got all my blm's in line after a few weeks of timing. I finally got everything running great. blm, pe, spark.....everything was great! I could hit the gas and go with no problems. I finally decided to take it out for a long drive. before that it was all just around my neighborhood. well after about 20 min of driving it started to stumble and stall while idling. I finally got it home by two footing at every stop light. I have since changed the rotor, spark plugs, tps, and iac. I have also done a iac re learn about ten times. still nothing. I watched the temp sensor with tuner pro and it stayed right in line with my mechanical autometer temp gauge. the only thing I havent changeD or checked voltage on is my map but have looked at it with tuner pro and it doesnt really look like the readings are off but I def need to do a little more in depth looking at it. Here is the strange thing.......while idling at around 700rmp its really rough and eventually will die but when I disconnect the cts it will smooth out and idle great, a little high around 600-650rpm but none the less it runs smooth and wont stall. I plug it back in and it will stumble and die.

I'm at a loss. does anybody have any ideas for me?

Ihanks
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Old 09-01-2011, 07:32 AM   #2 (permalink)
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I had a similar problem a few years ago that ended up being a bad EGR valve. I deleted it & haven't had any problems since.
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Old 09-01-2011, 07:46 AM   #3 (permalink)
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I had a similar problem a few years ago that ended up being a bad EGR valve. I deleted it & haven't had any problems since.
Ahh I forgot to mention....I don't have an egr. I also have it shut off in the ecm. No emissions what so ever.
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Old 09-01-2011, 07:48 AM   #4 (permalink)
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Iv'e also checked for vaccume leaks but I kinda ruled that out by it idling well when I disconnected the cts.
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Old 09-01-2011, 08:32 AM   #5 (permalink)
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id start with the simple stuff before just replacing everything, take ur filter off and just make sure u have a good spray pattern, if u do then id check for vacume leaks. Ive had this kind of problem a few times over the years one time it was the mix of leaks with clogged injectors. The other time it was low fuel pressure form the fuel pump.
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Old 09-01-2011, 08:42 AM   #6 (permalink)
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id start with the simple stuff before just replacing everything, take ur filter off and just make sure u have a good spray pattern, if u do then id check for vacume leaks. Ive had this kind of problem a few times over the years one time it was the mix of leaks with clogged injectors. The other time it was low fuel pressure form the fuel pump.
yup thats pretty much the direction I went. The fuel filter wasn't dirty but i cleaned it anyway. I checked for vacuum leaks and that checked out ok. i just got a FP gauge in the mail so Ill hook that up tonight but I'm sure its ok because it doesn't have any problems when its above idle nor does it have any problems when I slam on the gas. the issue I'm having a hard time with is why does it idle fine when I disconnect the cts? i did an ohm check on it when it was at about 160deg and it checked ok. I also watched the temp on the scanner and the temp on my mechanical gauge the whole time it was warming up and it was within a few deg the whole time.
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Old 09-01-2011, 09:43 AM   #7 (permalink)
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Which comp cam did you put in it, number?

Did you reflash the prom for that cam? Depending on lobe separation you maybe running rich or lean while it still looks good on the O2. I'm guessing lean since it runs better with the cts unplugged.

Verifying the inlet and outlet fuel pressure would help too.
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Old 09-01-2011, 10:31 AM   #8 (permalink)
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Which comp cam did you put in it, number?

Did you reflash the prom for that cam? Depending on lobe separation you maybe running rich or lean while it still looks good on the O2. I'm guessing lean since it runs better with the cts unplugged.

Verifying the inlet and outlet fuel pressure would help too.
I dont remember off hand which cam it was but as I have stated before, I have tuned it all in over the last two months and it didn't have any idling problems before i took it out for a long drive.

I appreciate the help but please read what i have wrote before you write a response.
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Old 09-01-2011, 11:58 AM   #9 (permalink)
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Iv'e also checked for vaccume leaks but I kinda ruled that out by it idling well when I disconnected the cts.
Actually, that would make me tend to think of a vacuum leak. Not saying it's the problem, but disconnecting the cts will cause the system to go richer(unless I'm backwards here, and I don't have my notes handy).

More fuel going to a lean mixture(caused by a vacuum leak) will cause the idle to smooth out, and rpm to increase.



Since it has a few miles on it now, re-torque the throttle body, intake, and anything else that may have changed once you spent some time up to temp.

I know some intake gaskets don't technically require re-torquing, but I find it to be a good practice.

Seriously, sounds like a vacuum leak to me.....



Finally, a TBI question posted by someone who has actually done some checks and can help with the diagnosis, rather than the typical "My truck does this, What do you think?" sort of bullshit that always gets posted.

Hope you get it figured out soon. Sounds like it might be stumping you right now, but you at least have a handle on things.
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Old 09-01-2011, 12:09 PM   #10 (permalink)
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Actually, that would make me tend to think of a vacuum leak. Not saying it's the problem, but disconnecting the cts will cause the system to go richer(unless I'm backwards here, and I don't have my notes handy).

More fuel going to a lean mixture(caused by a vacuum leak) will cause the idle to smooth out, and rpm to increase.



Since it has a few miles on it now, re-torque the throttle body, intake, and anything else that may have changed once you spent some time up to temp.

I know some intake gaskets don't technically require re-torquing, but I find it to be a good practice.

Seriously, sounds like a vacuum leak to me.....



Finally, a TBI question posted by someone who has actually done some checks and can help with the diagnosis, rather than the typical "My truck does this, What do you think?" sort of bullshit that always gets posted.

Hope you get it figured out soon. Sounds like it might be stumping you right now, but you at least have a handle on things.
thanks! and I do agree with the vacuum leak but i have checked twice. i disconnected all the lines individually and sucked on them to make sure they hold vacuum and sprayed the whole thing down with carb cleaner with the iac disconnected. now I know that doesn't mean I didn't miss something but my blm's is right at 125-128 or so while its idling rough.
would it be possible that there is something wrong with the map? it almost seems like timing could be changing via the ecm but I'm not sure. I have a pretty good grasp on tbi but I'm definitely still learning.
how about the pcv? is it pretty much supposed to be open or should it be somewhat restrictive? I sucked through that too and it was really easy to pull air through.
Yea i didn't want to post a thread where the only answer i got was "do a damn search" lol. I made sure to cover that first! haha
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Old 09-01-2011, 12:33 PM   #11 (permalink)
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thanks! and I do agree with the vacuum leak but i have checked twice. i disconnected all the lines individually and sucked on them to make sure they hold vacuum and sprayed the whole thing down with the iac disconnected. now I know that doesn't mean I didn't miss something but my blm's is right at 125-128 or so while its idling rough.
would it be possible that there is something wrong with the map? it almost seems like timing could be changing via the ecm but I'm not sure. I have a pretty good grasp on tbi but I'm definitely still learning.
how about the pcv? is it pretty much supposed to be open or should it be somewhat restrictive? I sucked through that too and it was really easy to pull air through.
Yea i didn't want to post a thread where the only answer i got was "do a damn search" lol. I made sure to cover that first! haha
Could still be a vacuum leak. Lots of places for air to sneak in besides hoses. Likely source would be intake gaskets. Even using the old ether trick to check isn't foolproof, because the leak may be between the runners and the lifter valley.
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Old 09-01-2011, 12:38 PM   #12 (permalink)
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Could still be a vacuum leak. Lots of places for air to sneak in besides hoses. Likely source would be intake gaskets. Even using the old ether trick to check isn't foolproof, because the leak may be between the runners and the lifter valley.
So if that's the case then how do I find a little bastard leak like that?
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Old 09-01-2011, 01:01 PM   #13 (permalink)
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So if that's the case then how do I find a little bastard leak like that?
That's why I suggested re-torquing the intake. I'm not saying it's the problem, but the issues started only after you had driven it and got it up to temp. It's a classic sign of an intake leak. Doesn't take any effort to re-torque.

another thing to check would be battery voltage at idle.
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Old 09-01-2011, 01:11 PM   #14 (permalink)
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That's why I suggested re-torquing the intake. I'm not saying it's the problem, but the issues started only after you had driven it and got it up to temp. It's a classic sign of an intake leak. Doesn't take any effort to re-torque.

another thing to check would be battery voltage at idle.
the intake has been on the engine log before i put the tbi on but your right, it doesn't take much effort to re torque the intake so i will definitely do that. it is a carb intake with the tbi adapter and i will check that too.

The batt voltage at idle is about 13-13.5vdc with a brand new optima deep cycle.
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Old 09-01-2011, 10:27 PM   #15 (permalink)
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dahoyle may be on to something. i think you have one problem masking another. your fuel pressure is very high, 18psi is way out. according to howell it should be about 13-15 psi (yes i just spent hours tuning in my own tbi setup with thier help). with the fuel pressure that high the injectors will spray more fuel than normal, and the computer would normally be leaning things out to compensate, but if you had a small vacuume leak it would do the same thing. what are your IAC counts at idle. they should be about 30. if they are way low look for a vacuume leak.
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Old 09-02-2011, 06:10 AM   #16 (permalink)
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Well I finally figured it out last night and to be completly honest i feel like a retard lol. after going over everything with the DVM and checking for air leaks I figured it must be a mechanical problem with the engine. Well for whatever reason a lightbulb went off in my head telling me to check the voltage to the ECM. 9.5volts! Ah ha! I traced it to a loose connection and started it up. Fixed! so after everything I went through it turned out to be something that took me two seconds to fix. Oh well, you live and learn. Now every time there is a strange problem like that the first thing I'm going to check is the voltage to the ECM lol.
Thanks for all the help you guys gave me! I'm back in action!
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Old 09-02-2011, 09:27 AM   #17 (permalink)
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Well I finally figured it out last night and to be completly honest i feel like a retard lol. after going over everything with the DVM and checking for air leaks I figured it must be a mechanical problem with the engine. Well for whatever reason a lightbulb went off in my head telling me to check the voltage to the ECM. 9.5volts! Ah ha! I traced it to a loose connection and started it up. Fixed! so after everything I went through it turned out to be something that took me two seconds to fix. Oh well, you live and learn. Now every time there is a strange problem like that the first thing I'm going to check is the voltage to the ECM lol.
Thanks for all the help you guys gave me! I'm back in action!
Figured you'd find it.

Nothing quite like having a clue, a little common sense, and the desire to do for yourself rather than showing up with your head up your ass, and wanting everyone to solve your problems for you. Props to you.
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