joe350

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!!

joe350

I hereby invoke my one TTT for the day. Anyone have any ideas?

89Z

I've been down this road before. It turns out I needed a new set of fuel injectors. I put in a set of the Bosch (Ford Motorsport) and it runs like new. Just because the resistance checks good, this only means the windings are not shorted or open, and the injector will open and close. Many times the failure mode is that the injectors will not seal properly and they leak in the closed position, especially when hot, thus causing the rich fuel smell in the exhaust. For about $200, drop in a set of the Bosch Injectors from Summit.

joe350

I've primed the fuel lines with the injectors off of the intake but still connected to the fuel rail with no drips. This is cold of course, but then again my car idles rough both hot and cold. This isn't to say that the fuel injectors are not the culprit, just adding information.

nameinuse

How is it blocked off?

89Z

Joe,
When I had bad injectors, my idle was a lot worse when the engine was hot than it was when the engine was cold. I also primed the fuel lines with the injectors off of the intake but still connected to the fuel rail with no drips when cold. Apparently when the engine and injectors were hot, they leaked worse. I never had the guts to prime the fuel lines with the injectors off of the intake but still connected to the fuel rail when the engine was hot for fear of a fire.

Since your idle is bad both cold and hot, your problem does sound different than the situation I had.

Just out of curiousity, have you checked the resistance reading of the coolant temp. sensor at various different temperatures?

joe350

No I have not checked the CTS. I suppose I'll check that soon. Do you really think it could make that big of a difference?

joe350

Oh, and to answer the question of how my EGR is blocked... with a blockoff plate.

nameinuse

A sturdy one with a gasket that seals good?
A big enough leak here can set the "Manifold Absolute Pressure too high" (don`t have the book handy) code and render the car totally undriveable.
A small one sure screws up the idle and low rev performance.

joe350

A plate of aluminum with RTV sealant on both sides. Adding the plate had no effect on the idle good or bad.

joe350

With the way the vacuum fluctuates at idle, it seems like theres one or more vacuum leaks below the plenum, but not on the outter seams. Perhaps my valve seals are worn to the point that it is drawing in air from the valve covers? Another possibility is that its leaking at the intake/head seam, but on the inside only.

joe350

Update. Replaced the timing chain with a small improvement of idle quality, but still not perfect. What do you all think... time to remove the heads and get a valve job? Could valve oil seals wear enough to cause a vacuum leak or would that have to be the valve guides themselves?

iroczracer07

iTrader: (2)

With the way your timing is fluctuating, I'd say your distributor bushing is worn out. Pull the distributor and check it for the usual.

joe350

Ran a few more tests. Base timing jumped from 6 to 8 degrees BTDC after the timing chain was replaced. No suprise there. Vacuum still sits around 19.5 in/hg and vibrates 1/4 in/hg each direction. Timing still retards slightly when throttle is increased a little and then jumps suddenly several degrees when throttle is increased a lot, EST wire unplugged of course. This makes me think the distributor is the culprit (for the timing discrepancy anyway). Anyone have anything they want to add?

iroczracer07

iTrader: (2)

joe350

Ok, the distributor may be bad, but how much of a difference could it possibly make besides throwing off timing?

iroczracer07

iTrader: (2)

Idle quality would suffer, gas mileage would suffer, performance would suffer, and so on and so on.

joe350

How?

iroczracer07

iTrader: (2)

I'm going to explain this in the simplest terms because I don't know how much you know about cars. Using a timing light to set your timing, you may find that your car will perform better at a certain point such as 6 degrees, just to use a number. So, you lock the Distributor down to keep the setting. If your buddy decided to play a joke on you, and loosened your Distributor, turning it to some unknown point, your car may not start. This is a simple point on which I'm sure we can agree. Now, using the above example as a reference point, if the Distributor itself, or the bushing gets worn out to the point of no longer being able to maintain a timing setting, your ignition timing would be all over the place. That means that the timing is in a constant state of flux. One second it's too far advanced, the next it's too far retarded. Either of these could lead from dieseling when you shut the motor off to detonation when you're driving down the street and everything in between. Since in our theory, you had already found the optimum timing setting, anything else would cause you to lose performance. Detonation is not only bad for your fuel economy, but it can cause internal damage as well. The fluctuating of the timing might also cause poor idle quality. Hopefully this explains the situation a little better for you.

joe350

So basically you are saying the distributor will only effect the timing of the spark, not the quality. When my EST is unplugged and the car is idling, the timing stays still and the rough idle is still present. This tells me that whether or not the distributor is bad, it is not the primary source of my rough idle. Would you agree with this assessment?

86BirdSE

Just curious... when is the last time you've replaced your ignition coil?
Plug Wires nice and tight, and not leaking spark?
How old is (and who made) your ignition module?
Are you living in an area that now has 10% ethanol? Chances are the new gas has "cleaned" your system and plugged your fuel filter. By law the pumps should have a sticker on them stating this fact.
You stated that the vacuum fluctuates slightly... How is your PCV valve? Do you have smoke on start-up?


Just a couple of things to keep your mind from going

joe350

Ignition coil is about a year old. Ignition module... who knows. Where is that located? PCV valve is fairly new. Of course it smokes on startup. It is a SBC after all.

joe350

Ok, I found a picture. It looks like the piece that sits on the distributor. What exactly does it do? Control the distributor's timing?

iroczracer07

iTrader: (2)

I believe the ignition module opens and closes the ground circuit in the coil to fire the spark plugs. If the fluctuation isn't happening when the esc wire is disconnected, then I don't believe it's your distributor. It sounds like the knock sensor might be sensing detonation. What octane fuel level do you use?