209z28guy

Hello all,

Posting this for my dad. The car at hand is a '88 C4 automatic that has a condition of running rough (slightly unstable idle) and chugs and stutters a little around 1200rpm. No smoke out of the exhaust no leaks. So now... heres whats been done to the car:

New rebuilt motor
Every (and I do mean every) ignition component replaced
New o2 sensors
New cooling system (all components)
New SLP runners with port matched lower plenum
BBK throttle body (tb bypassed)
All sensors have been replaced from the knock sensor up
No vacuum leaks anywhere
No oil leaks
New fuel pump/filter/FPR/Southbay 22# injectors/ o-rings/all pressures are correct at all rpm ranges
No ECM codes at all
New computer
New AC components all around

Long story short this issue has been ongoing now for 2 years and has been unsolvable by 4 different shops. One shop i worked at and even had this thing hookup up to a live scan tool and all parameters looked normal. The car runs slightly better in closed loop but still has a stumble at idle and IAC has been cleaned passages, new IAC and has been reset properly.

Heres where it gets a little weirder, if you squirt a little maf cleaner/tb cleaner/carb cleaner into the air filter or around it and it runs smooth as silk in either open or closed loop (ruling out o2 sensor operation or maf issues)

From what i can tell, my best guess is that either he has the wrong injectors (may have got 19# injectors by mistake), 9th injector might be leaking and stuck open?, or the last possibility is that the wiring harness (that has been gone through at least twice) has a short somewhere or has high resistance somewhere... my dad, myself, a 25 year GM tech, and 4 other 10-20 year diagnostic techs have been banging their heads on a wall trying to figure out this issue. There is no aftermarket cam and the rest of the motor is built to the tee to factory specs... if anyone has any guesses or input into what it could be or into my guesses would be massively helpful. Thank you

Kevova

Spraying making it better sounds like slightly lean. INT/BLN ? Do you have access to dealer test equipment? You could try adding braided ground straps...GM's intermittent fix all back in the day.

Cliff Harris

Your description is the classic symptoms of a vacuum leak.

209z28guy

Hey Kevova,
It does appear to be lean. However it just began to seriously sputter, no CEL. INT/BLN? no access to dealer test equipment. And as far as ground straps I THINK these were reinstalled when the motor was rebuilt but not sure. Where would these normally be located? I wouldn't think one grounding strap would cause this much trouble but maybe??? At this point a grenade seems like a better option... lol

209z28guy

Hey Cliff
Checked everywhere for vacuum leaks. No evidence of any leaks. Ive considered that the remanufactured MAF sensor could be a possibility, but it has been functioning within its normal limitations.

ddahlgren

Unplug the injectors one at a time and find the dead hole. I am not sure how 4 diagnostic techs can't check BLM/INT as my old AutoXray scanner can. My money is on a leak at intake or runner gasket.

mlm0

Have you monitored your fuel pressure at the time it runs bad?

209z28guy

Unplugged all injectors as mentioned. All working normally. All intake runner blots, gaskets, and mating surfaces are 100% or new. 9th injector maybe... but im leaning more towards a wiring or grounding issue

209z28guy

It runs bad pretty much all the time. Fuel pressure is good. New injectors, FPR, pump, filter, and all associated hardware and gaskets. The only thing not replaced was the 9th injector, which i know i have a block off fitting for the rail but it ran bad before that too? The car also just got a freshly built trans in the midst of all of this. No change in running condition.

kenmohr

check your timing. Mine was set at 6* after tdc. It would shutter under pressure. Someone misread the rotation.

209z28guy

timing is 100% on point. And timing has been set using the correct method of disconnecting the connection at the distributor. I wish it was just timing. Haha

ctmccloskey

One more thing that you might want to check is your Exhaust Re-circulation system (EGR) on your car. My 1988 L-98 started running rough with a poor idle and I tuned it up and replaced a lot of parts before it turned out to be a plugged EGR system. There was so much carbon in my intake manifold that there was no way for anything to work properly around the EGR parts. I had to remove and clean out the whole system and install a new EGR and Fuel pressure diaphragm with all new gaskets. The hardest part was understanding how the EGR Solenoid failed like it did. Mine failed open with a broken hose and I ended up with a major vacuum leak. I never got a code for any failure
or problem.
This is a device that can effect your idle and really pushes the emissions through the roof.

In Virginia they test our older cars on a Dyno and at various speeds and when my EGR was acting up my "NOX" went way out of specs. Once I saw the emissions data I knew where to start looking. Maybe big brother actually helps some times. Good Luck!

tomcamaro97

how are you checking vacuum? i kept checking mine by smoking my intake using my brake booster line.. low and behold... my brake booster was leaking , so even though i was smoking the intake, i wasnt testing the one thing that was leaking..

did you block off the 9th injector? i know you said you had the fitting laying aroudn.. ide throw it in, since its cheap or free and rules something out. what is your fuel pressure.. does it stay steady as you rev it? unplugged the regulator? fuel pressure staying after you shut off the engine?

probably not the icm.. but they are a known common problem.. and once again for $15 can rule something out.

VikingTrad3r

can you verify that the chug is only at 1200 rpm?

or did you mean that the car is idling at 1200rpm and has the chug? or soemthign else.

did you use a piston stop to verify that the harmonic balancer mark is \in the right spot? not sure if they gave you a new one when it was rebuilt. problem is that you need to check everything so even if they did, i'd be doing that and verifying its actually at 6 deg when you think it is


egr....i agree with cliff in that it sounds like a vacuum leak, and it sounds like it might be an egr vacuum leak. i think you should check to see that the pintle on the egr valve is seated perfectly flush on the egr valve. might be some exhaust sneaking around the pintle.

its also possible that the egr solenoid has failed slightly open, allowing just a bit of vaccum through the vac hose and causing the pintle to slightly lift off the seat again allowing exhaust into the system when it is not being called for. for this test, pull the vac hose off the egr nipple. if it solves the issue then the solenoid is stuck slightly open.

but before you do that, when you go stand behind the car while running, does the exhaust smell really bad? not burning oil bad but like metallic/caustic/make your clothes stink for a while bad? if that is the case, that is the high nox caused by the egr allowing some warm exhaust into the intake when not called for. it creates a hotter combustion chamber at the wrong time and creates more nox which can be smelled out the tailpipe.

your blm/int will tell you if the csi is dumping fuel in.

mlm0

what is best way to check brake booster for vacuum leak

RWDsmoke

I fought my rough idle for 10 years. Replaced everything above the block, except the intake manifold. Had it off several times, new gaskets, still rough idle, smoothed out above 1500 rpms. Could't find any vacuum leak, but still symptoms of a vacuum leak. Finally replaced the manifold after reading this. http://www.powerperformancenews.com/...manifold-seal/

Now it idles smooth. I had no oil consumption that I could even notice, only a couple of spark plugs that were a little darker. Really hard to find that vacuum leak on the bottom side of the intake.