[Poda]

I've been having an ongoing shudder at partial throttle under load for a while now. Initially I thought it might be a TCC lockup issue, but the tranny and TC have been swapped out since August for a different reason, and while she shifts like a dream, it did nothing to alleviate the shudder.

I know I sometimes trip Code 32 (EGR valve) which I haven't addressed yet, and since the tranny swap I've got a lean condition on the driver's side o2 sensor thanks to a badly re-installed gasket upstream.

However, I've had this shudder since before both those issues..

The log was taken on the highway portion of my commute to work, and the first shudder starts about a minute in.. another one about 30 secs to a min later, then a couple more throughout the run.

It's pretty easy to spot where it's happening - spark retard jumps through the roof.

So yeah, I know the basics of what to look for, but any gurus want to give it a shot and maybe point me in the right direction to fix it once and for all?

https://www.dropbox.com/s/423i64ktpjn9mkn/94 Cad 12-02-13.uni

 

[bobdec]

Looks like your missing (mini stalling) while running along, in pic below 1 min into log you were running at 47 MPH and tapped throttle to 18.8% TPS. But instead of RPM's comming up they dropped, it went a bit lean, RPM's dropped, injecter pulse width increased, MAP went way up (lost vacuum) and caused you to enter low RPM/High MAP cells in main timing table that eventually dropped timing to 12*. The KS then picked up knock (could be the shaking engine) and dropped timing another 4.8* so you ended up at 8* timing. Throttle then stayed steady at 19% RPM started back up, MAP went back to normal, and 3 seconds later,the event was over. Could be ignition, fuel starvation related hard to tell. Although you are running a bit lean nothing on the log indicates bad sensors or dramatic problems. Unfortunately the OBD1 does not monitor HV ignition/wires/plugs or fuel pressure. Your LT BLMs show a lean overall condition they are pretty high in the 140's at times, but the ST BLM's (actual reading) show the PCM is correcting, not perfectly 128-135's but it's doing as best it can.

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[Poda]

Thanks for the analysis, it's greatly appreciated!

A couple of things I've noticed over the last month..

- Vacuum pressure seems sporadic sometimes. For example, I'll be stopped at a light, consistent pressure on the brake pedal, and I'll feel it drop down a bit. Engine's not missing when it does that though, at least as far as I can tell.

- I'm throwing code 32 on a random basis. To paraphrase, "No change in A/F mixture when EGR commanded to open". The solenoid's been replaced with a known-good, and EGR valve is next. Even if it's not the issue, it's original, and probably gunked like crazy.

Both of these could probably be explained by the loss of vacuum pressure you saw when the engine "mini-stalls", and could be a symptom as opposed to a cause

- I was planning on doing plugs/wires when I swapped in the new heads in the spring, but I may do them sooner if there's a viable chance they're the issue. Coil and ICM are new as part of a previous issue, and spaced away from the head.

- Fuel pump was replaced a month ago. Original pump died on me, replacement pump was defective and fried my harness, and the third seems ok. However the car is now hard to start when cold, and I'll be picking up a pressure gauge on the weekend to see if that work needs to be re-done.
It could still be a fuel delivery issue, but the car was behaving this way long before the pump died on me so I'm skeptical this is the root of the problem, rather something that would potentially compound it.
Fuel filter is 6 months old, so I'm doubting that it could be that easy.

- Lean condition can probably be easily explained by my exhaust leak upstream of the o2 sensor

So with all that said, I'm thinking the next course of action would be...

- Replace vacuum lines (I wanted to go silicone anyway, and seems an easy job)
- Ensure fuel pump isn't making it worse with low pressure
- Plugs/Wires
- Replace EGR valve

Anything else you guys can think of that would help narrow it down so I don't spend a freezing winter day on a weekend in the driveway doing this stuff?

 

[bobdec]

Had a little more time today and looked at you log at other points besides the shuddering spots. First off your PCM think's OR knows you are running way lean and the PCM is adding a ton of fuel. I sat thinks OR knows because you have exhaust leaks, At idle in the front of the trace your LT BLM's are 160 and that's top display they could be higher. That says your PCM is adding 25% or more fuel. You have to fix the leaks you talk about first before you can get valid log data. Also you IAC at idle is only at 9 it s/b higher up near 20 or more for good idle control. Tour TPS voltage is 0.61 crank in the stop to 0.55 volts and see what the IAC counts go to. My direction towards tuning is 1st get the idle correct, then a running street tune, after that performance. Since your idle fuel mix is messed up that's first to look at. Can you post a tune (.bin) file, I would like to see if that's causing the lean condition. Your idle at idle looks OK at 39 Kpa so no major vacuum leaks, mine idled at 35 Kpa when it was stock (w/o cam). PCM adding fuel due to an incorrect O2 reading could cause rich burn and that could plug EGR valve. At idle if you pick valve w/a mini vac pump or using your finger to activate plunger does the engine loose RPM, it should if EGR path is not obstructed..

 

[Poda]

Here's a link to my most recent BIN, which I pulled the day before I took the log (just got my cable..)

https://www.dropbox.com/s/26to3xroxtum012/1994 CAD.bin

It *should* be bone stock.. I haven't done anything with it yet.

The engine does drop in RPM's when I activate the plunger, so that's good to know and one less thing to look at.

I really appreciate the help and advice. Looks like I'm going to have to brave the cold and fix that exhaust leak ASAP. AFAIK, the gasket at the driver's side collectors is the only place it's leaking, although I do have a couple of sheared manifold bolts too (rear bolt on either side), but I'll make sure the others are torqued to spec.

 

[steveo]

I'm throwing code 32 on a random basis. To paraphrase, "No change in A/F mixture when EGR commanded to open". The solenoid's been replaced with a known-good, and EGR valve is next. Even if it's not the issue, it's original, and probably gunked like crazy.

i believe you're misinterpreting that code -- every gm pcm i've seen tests EGR operation by checking for the expected drop in manifold vacuum when the EGR opens. a stuck open or clogged EGR would present no change in vacuum when opened.

this can be set falsely by all sorts of things, including a sizeable vacuum leak.. since that vac leak would overpower the EGR's minute drop in vacuum...

 

[Poda]

Thanks Steve - you're right, it is checking for change in manifold vacuum, not A/F mixure. My bad..

 

[steveo]

you can remove the valve and apply vacuum to it to check its operation.. if it's working properly, that EGR code could be a really good hint that something causing vacuum weirdness is going on.

unmetered air is nasty stuff, and im pretty sure your map kpa is a bit on the high side for a stock engine. mine is cammed and it didnt pull 39kpa at idle even with a leaky brake booster.

 

[Poda]

So assuming vacuum weirdness is going on, what should I check next? I wanted to replace all the hard vacuum lines anyway so I guess doing that first, replacing PCV valve (unrelated I'm sure, but dirt cheap while I'm replacing hoses), and fixing exhaust leak are the first things to do..

 

[steveo]

i would check each vacuum line as you go, with a hand operated vacuum pump, because you could be putting a fresh vac line on a leaking vacuum accessory, and for your own peice of mind that you're actually repairing something that's broken.

then i'd spray around the intake just for kicks, to make sure you dont have any vac leaks there (also bottom injector seals can vac leak without spraying out gas)

might as well

 

[Poda]

What should I be spraying around the intake, and what should I be looking for?

I'm pretty mechanically inclined and fearless when it comes to replacing stuff (did the tranny swap in the driveway with the car on jackstands.. never again, but that's a different story. I learned there that renting or borrowing space with a hoist is worth it), but troubleshooting isn't really my forte.. yet.. Apologies for the basic questions.

 

[steveo]

water...carb cleaner.. propane.. whatever

if you hit a vac leak around a gasket your engine will change rpm noticeably.

 

[bobdec]

Bin file is OK, stock cyl volume and injector settings.

 

[Poda]

Thanks Bob - that's what I thought..

For the TPS - my (albeit limited) understanding is that it's non-adjustable.. How do I crank it down to 0.55v?

 

[bobdec]

LT1 gets idle air two ways 1-small amount through blades in TB, and 2- the rest through Idle Air Control valve (IAC) that feeds idle air plenums in intake manifold, the plenums deliver air to each cylinder evenly. The PCM controls the IAC using 'counts' (IAC counts) that you see on a log. That's how idle speed is controlled. The counts s/b between 20-40 so the computer has room to adjust idle up and down by stepping the counts as needed. You are running IAC count of 9, saying your getting minimum air through the plenums and the rest from TB blades. If closing TB blades does not increase IAC enough then you may have a small vacuum leak. If you close the throttle blades using the set screw the Throttle Position Sensor (TPS) voltage will drop, the spec on TPS is 0.45 to 0.75 volts, so going to 0.55 will be OK. As you close the blades the voltage will drop and the PCM will increase the IAC count allowing more air through the idle plenums. That helps smooth out the idle and acceleration off idle by delivering more air to each cylinder rather than a blast or air coming from way up front. I will reiterate this will not fix you rotten AFR readings at idle, you need to get that resolved also.

 

[Poda]

Awesome - thanks Bob, that clears things up a lot. I've got a garage to borrow on Thursday, so I'm doing my exhaust leak and testing for vacuum leaks then. Hopefully, that'll go a long way to fixing AFR at idle.