Hey guys, I've got an interesting issue on the '88 irocz I bought last year. It has progressed and gotten worse in the past few weeks, so I figure it's time to take care of it. The car was a single owner, 114k miles, maintained well, mostly garage kept. It's all original, not a single modification, all factory options still on the car, and it's a 5 speed manual.

When you cold start the car, give it a minute to idle down, then take off...if you don't feather the throttle in, the car will violent jerk and the RPMs will fluctuate like it's not fueling properly and can't climb. Eventually it'll shake it out and climb easily past 2k rpm. It's incredibly apparently in 2nd and 3rd gear if you stab the throttle before the car is up to temp. Once everything is warmed up it'll still be there, but less violent.

I scanned the car and noticed the CTS was wrong on cold starts...replaced it. The values are correct now, and cold start has improved...but the jerking still persists. Since the IAC was original, I replaced that as well. It was pretty dirty, and it idles better now (i did min air procedure too), but the jerking still persists. I've used carb cleaner probing for vac leaks, everything is perfect, not much rubber hose cracking after all these years. Today I used the ol' DMM to check the coil, injectors, TPS and had the ign module tested. The scanner shows IAC counts are good, and the MAF checks out, as well as the MAF relays. I even unplugged the MAF so it'd default into LHM, didn't change the bucking. Cap and rotor are making good contact as well, wires and plugs also check out.

A coworker of mine worked for GM during the late years of thirdgen and C4 development, and he suggested the EGR might be the culprit. I suppose after all these years it might just need to be replaced, and has really degraded recently. What do you guys think? Have you ever had a stuck EGR cause this much grief?

 

Check the fuel pressure & bleed down rate.

 

Checked, everything was within spec. No serious amount of bleed down.

Any other ideas from anybody?

 

Does the SES light flash on as it is bucking/jerking?

 

Hi Mike, Unfortunately no it does not. I also tried to pull codes..zero codes! I figured it was EGR related after all my diagnosis because these early cars used the diaphragm type EGR(versus later model cars using the solenoid type), which had a weird feedback loop to the ECM to let you know the EGR wasn't quite working right... That's just a hunch, based on things I've heard people say

 

On a cold start, it'd take some time for the temp to get up enough to control the EGR.
If the EGR pintel isn't seating, or the diaphragm is leaking, you would have a hard time starting the car, as it would act like a vacuum leak.

I can't see anything in the EGR system that would cause the engine to buck, especially at higher RPMs.

Does it jerk/buck when revving in park without a load on the engine?

 

Bump. I have the same problem. Seriously thinking about converting to carb.

 

I think 8Mike9 might be on to it. Could be transmission, T converter or transmission electronic control related rather than engine related if it revs freely while stationary.

 

Hey Mike, I follow ya. If the car is cold started, and i put it in neutral and stab the throttle, it'll choke and stumble, eventually clearing up above 1500rpm or so. So yes it will stumble/jerk without load.

The engine doesn't buck in higher RPM like you stated, it's only lower rpm mostly off idle.

 

Have you ohmed out your plug wires?...I'm still thinking ignition here.

One other thing...but usually shows itself at higher RPMs (my mistake in reading your OP) is a PU coil.

The PU coil can be ohmed out (do it while wiggling the wires) IIRC they should sit right about 800 ohms and not fluctuate...also look for corrosion in/on it, as well.

I know you mentioned the cap and rotor look good, but do you know how long it's been since they were replaced?

And lastly (but maybe should have been first) is what do the plugs look like?

Do you get a puff of oil smoke on a cold start?-i/e bad valve stem seals.

I'm wondering if you have some oil getting into a cylinder or two and lihtly fouling a plug, yet after running, burn off enough to allow the engine to run well.

The offending plug(s) will still show signs of the fouling.

 

Hey Mike, great ideas! I'll do them and report back soon.

 

Hey guys, I apologize for the long day, but work/life priorities shifted for a bit. I did some investigation per Mike's suggestions above and found the following:

I've ohmed a few of the plug wires and they're reasonable so far, I will finish them soon and post back with all of their values.

I've researched symptoms of failing PU coils and found the test procedure for mine. It indeed reads infinite resistance grounded from each of the 2 pins, and checks out at around ~850ohm between the two terminals while I wiggle the wires and check for opens. However, I've attached a pic of my pickup coil as it sits. It's VERY rusty and probably quite old. I wanted to inspect it more but do you think it's just worth replacing at this point?
Pickup-Coil pic

Regarding the cap and rotor. I know I looked at them briefly before, but I decided to inspect them closer this time. Somehow I missed that the carbon button was totally gone! I shouldn't have missed it the first time... but in any case I've replaced the cap and rotor which feels like it helps the car during operation. (probably due to spark timing lag as it needed to build electrical potential to jump yet another gap without the carbon button in place?) However it doesn't appear to help the bucking sadly.
My bad cap is on the right.

Next, I removed all of the plugs this time, as the first two I checked last time looked pretty good and I didn't invest the time to remove the hard ones . I'm not sure the last time they'd been changed, but they looked quite old. They were indeed delco 45TS plugs but just look like long-life wear. Nothing seemed too out of the ordinary or fouled (even tho i have some stem seal issues and oil burn on startup). I did find that cylinder 6's plug had cracked porcelain! So I figured new plugs would make a difference, unfortunately the bucking issue still persists!
Plug Wear
New vs Old plug

So, while the car is better during operation, it still bucks on cold start takeoffs. I dunno if I mentioned before, but on occasion it'll start right up with the twist of the key, other times it will crank for 4 seconds or so before slowly catching and then taking off...like it's intermittently weak. I thought for sure these ignition parts would help but it still persists. Could a failing PU coil cause these start issues and bucking ignition problems? If it's that rusty and old, it could be suspect regardless of the good values. My fuel pressure and bleed-down values were good, but should I just yank the injectors and clean them at this point?

Lastly, I've finally got a laptop and can datalog with TunerProRT. I've got 1 cold start acceleration log so far, but it didn't exhibit the problem bad enough...once I get a clear datalog showing the issue I'll post it for review. Thanks!

 

bump, any ideas?

 

that's a lot of rust to be having in a distributor. it would be something I would change for that reason alone

 

Ya, that reluctor looks bad, I think a new one is in order.

 

So I replaced the entire distributor but the bucking was still persistent! I was pretty frustrated and started thinking about the fuel system. I managed to get a very clear datalog of the issue and showed a friend. He coded an arduino datalogger and we used the I/O for high-speed data logging the fuel pressure at the rail. We noticed some fluctuations and decided to change the fuel filter to start. I know it was last changed in 2007, but figured it'd be worth the shot.

Long story short, it seems to have fixed it! It also fixed the long crank issues I was having...and so far I haven't had bucking issues on cold operation. I've attached a picture of the cracked-open old filter. Looked pretty dirty. I appreciate all the help on this guys, hopefully it serves as an anecdote for anyone in the future!