91-Z28-L98

Like it says, I got a rough idle problem. Car is a 91 Z28 with 350 TPI engine. Idle stays rough around 650 RPM. With the IAC closed and unplugged the idle is wild, ranging from 500-900 RPM with frequent stalling.

IAC, throttle body, and upper intake have been cleaned and seem to have no problems. I looked thoroughly and cannot find any vacuum leaks. I used both the "hose to my ear" and "propane at the seams" tests to no avail.

The only code being thrown is the code 32, faulty EGR, which is thrown at highway speeds. This is because I have blocked off the EGR, so it is no suprise.

Timing is good, set at 6 degrees BTDC. With the EST plugged in the timing moves a bit at idle, between around 18 to 24 BTDC. Don't know for sure. Rough idle is persistant even with the EST unplugged though so I don't think thats the problem.

ECM is sending a good 5v reference. MAP and TPS have good base (idle) readings and rise steadily with vacuum/throttle change respectively. O2 sensor is fairly new.

Fuel pump, filter, and pressure regulator are new. Injectors are used, but work far better than the stock injectors I previously had. (2 were bad, used the resistance test to check them) These injectors all pass the resistance test.

EVAP canister is blocked and the vacuum port is plugged. PCV valve is fairly new.

Plugs, wires, cap, rotor and coil are all new.

Compression on all cylinders is between 190-200 psi.

Vacuum vibrates between about 19-20 in/hg at idle. This hopefully is an indicator of something.

Standard SBC smoke on startup happens. Otherwise I only smell rich exhaust. I lose no coolant or oil and my compression is fine so I don't believe I have a headgasket problem.


Any help on this would be greatly appreciated. I've been dealing with this for a very long time!!

V8Rumble

Only thing that comes to mind right off the bat is maybe a stretched timing chain.. (Good excuse to upgrade the cam, right?)

I'm thinking that it'd also be a good idea to alter the ECM code so that it's not expecting the EGR, but I'm not a tuning wizard...

Good luck with it.

91-Z28-L98

I had that thought as well, and at 120k miles I'm sure its stretched some. When I check my base timing (EST wire disconnected), it is about 6.5* BTDC at idle but when I increase throttle it slowly retards to about 5* and then at a certain RPM it suddenly advances to about 14*. I'm told that the distributor automatically adds advance without computer control at a certain RPM, so I guess thats normal. I figured that a stretched timing chain would cause a less stable base timing though. I'm no expert on these things, so thats why I'm posting.

I just want to have some good evidence pointing toward a certain part being bad before I start replacing anything I suspect, as I have done in the past. The only possible problems left are too expensive and take too long to replace and have it turn out not to be the problem after all.

V8Rumble

Well, that strengthens my suspicions about the timing chain...

That's good thinking. And it is indeed probable that the distributor could add some advance with increasing RPMs, I'm sure you're familiar with the weights & springs used in pre-computer distributors. But (unless my understanding of how they work is way off), there's NO WAY that that system should even be able to retard timing - it should start off at the base timing, & then add timing via centrifugal force.

My best guess - still waiting for all of the coffee to kick in - is that you're seeing a minor version of a "cascading failure". The timing chain is allowing for more slop than should be present, & the ECM is doing what it can to try to correct for it.

But, I'm just getting back into working on cars again after taking a 2-3 year hiatus, so hopefully someone w/ more-recent experience can chime in here...

Evil TA

sounds like it may be a timing issue of sorts - the first thing i would do is try a known good distributor, just to rule that out...

also, IAC is only 30 bucks... might be worth replacing...

91-Z28-L98

The IAC is the only thing keeping my car from stalling. With it unplugged (from diagnostic mode) the idle is crazy and lasts about 10-20 seconds before stalling. I doubt the IAC is the problem.

I've asked this on other boards as well, and they also list the distributor as a possible suspect. Would the distributor have an affect that would hurt performance besides causing bad spark timing? I ask this because idle is bad even when the spark timing is sitting at 6.5* BTDC or so (EST wire unplugged).

I don't have access to any test distributors unfortunately. They're only like $100 but thats expensive to replace something that may not be broken.

I really do appreciate all the input. Thanks guys.

irocalot

I'm thinking timing chain myself, but if you want to do a quick check of the distributor drive gear, that wouldn't cost you anything but time. On my 88 with 120k miles the teeth had worn down to razor sharpness - but it ran fine, other than the knock. Maybe a tooth on yours is totally gone? The bushings inside could be bad, for sure at that mileage, but the timing chain is alot more common, and certainly has worn, unless it was changed by a previous owner.

You can test the timing chain for slop by pulling the distributor cap and turning the balancer - while watching the rotor - You'll be surprised how much slop there is in an old chain.

91-Z28-L98

I have pulled the distributor before when I replaced the intake manifold gasket. It looked fine then ( a year ago or so) but then again I wasn't checking it really.

89Vert

I used to have a rough idle, turned out it was the inductive pick up on my distributor,
but you need a graphing multimeter to check it

91-Z28-L98

So maybe I'll replace the distributor next if the chain doesn't fix it.

ss00

I had the exact same situation on my '89 Iroc and went through the same diagnostic steps you are going through. Long story short......it turned out my injectors were not sealing properly when hot. Remember, the resistance test only tells you if the electromagnetic coil in the injector is open or shorted. I replace the injectors with a set of Bosch from Summit (about $220) and it runs like new.

DCJLove

I find this exchange interesting, but am wondering how an old/stretched timing chain would affect my carbed 305? I notice a pretty rough idle when the motor is warm. I get a bounce on the tach between 650 and 850 rpm at idle. Would it exhibit this behavior from the chain wear or would it likely be a carb adjustment issue?

91-Z28-L98

Well, a way to test for this is to test the fuel pressure with the engine hot. If the pressure drops quickly, then you are right. I'll try it though.

raul.garcia

The retard is from the ECM testing the Knock sensor on startup.
Without ECM control the only advance you should see is the amount you manually set.

You probably won't believe me if I tell you that an Electronic Ignition System can't add timing of it's own so, here's an excerpt from a thirdgen Tech Article unless you don't believe them either.
"Non-Computer Controlled HEI Setup For Performance
Damon Mar 31 2006 - 9:05pm"

Computer controlled versions work basically the same as the old non-computer versions except that they don't have any mechanical advance mechanisms inside them- the ECM determines the advance curve electronically. There is NO WAY to change the advance curve of a computer-controlled HEI distributor- timing is controlled ENTIRELY by the ECM’s programming! The only change you can make in the advance curve, other than buying an aftermarket performance chip for the ECM, is to manually advance the base timing.

89Vert

the inductive pickup is what tells the ecm when to fire the spark,

on mine the signal was good but weak at idle causing my timing to be erratic with the timing wire hooked up. it idled fine with it disconnected to set base timing,

a rough idle could be caused by many things, how rough are we talking?