My car runs a bit rough at idle, even when engine is warmed up. Has a new distributor cap and rotor, wires, and plugs with verified gap. I checked base timing and it's right around the 6 degree mark as it should be with the ESC disabled (and in this state the idle is even rougher). With ESC re-enabled at idle I notice it is advanced pretty far - it looks like 15 degrees or more (it's well above the indicator). Is this normal?

Power also seems weak and there's a ticking sound under load. I'm trying to find the next likely culprit. Listening under the hood I hear a possible vacuum leak sound (to me it kind of sounds like a rubber plunger pumping fast) when it's running, seems more pronounced on the passenger side, kind of centered somewhere between intake manifold and distributor area. I haven't replaced all vacuum lines yet so I think disassembling the intake and inspecting/replacing all those lines that will be my next task.

It could also be an exhaust leak I think. If so it's on the passenger side. But the noise is less pronounced at higher RPMs so that doesn't seem to corroborate that idea...?

Anyway, apologies for the miscellaneous imprecise descriptions. Thoughts are welcome.

 

What year? It could be leaking vacuum through the brake booster...does the hissing sound happen when you push the brakes?. I had that happen. Also check the fuel pressure regulator. Pull the vacuum line off and check for gas liquid or smell. If so the diaphragm is bad. That will make it run bad.

 

This is a 91. It doesn't seem to be coming from the brake booster, as far as I can tell. But I haven't tried pressing the brakes. When braking while driving I don't notice any correlation, but not sure if that's relevant. Will check the fuel pressure regulator - the noise does seem to be in that general vicinity.

 

Could it be the EGR Valve working? It sits up there and gets pulled and released pretty often IF your EGR Vacuum Solenoid is using Vacuum to control it. You should be able to move it with a hand held vacuum pumps and you will hear a difference in the engine.

PCV Cap clicking maybe?

The cruise control vacuum connections are what caused me grief. I cut off 1/2" off the hoses before finally replacing the whole set. The 1/2" is frequently just enough to make the old hose work a bit better.

It be interesting to see what the Vacuum is doing in your engine while the engine is idling.
If you want to really get the performance back you might try a new Oxygen sensor and replace the basic tune-up parts. I would suggest a bottle of BG 44K Fuel injector system cleaner through the gas tank and see how fast it clears up and smooths out the idle. It's not cheap but it is very effective.

If the problems are still there you will need to find a OBD1 scanner that will connect to your Corvette.

 

Focusing on checking EGR system and fuel pressure regulator tonight - hope one of these is the key! Really excited to get this thing purring like a kitten (and roaring like a lion!)

 

OK, stupid question. In the FSM I keep seeing the instruction to "ground the diagnostic terminal". For example to begin testing the EGR solenoid. Does this just mean to ground pin B on the ALDL?

 

Never mind on the stupid question - I figured out that my assumption was correct.

EGR solenoid is good (and I'm glad because the damn things are not common, and expensive).

EGR vale is BAD - won't hold vacuum. So I have one of those incoming today. Given the solenoid was blocking vacuum during idle it likely wasn't affecting rough idle, unless it is stuck open, which I won't know until I get it out. But it definitely could have caused the noise I hear when under load, if it's spark knock.

Going to test fuel pressure and regulator later today (didn't have a gauge, and that's also incoming today).

To all the recommendations I've seen about getting the real FSM, couldn't agree more - the diagnostic charts are fantastic!

 

Good Work Incepheus!

You have found a bad EGR and that is a good thing. The EGR like the fuel pressure regulator has a rubber diaphragm in them. Any time Ethanol and Rubber meet it is bad for the rubber. The EGR valve will affect your engine when broken. I had an EGR go bad and I learned of it when I had my emissions tested. When the EGR is bad it makes the engine run hotter Cylinder Combustion temperatures and this leads to needing High octane fuel to keep it quiet.

If you are going to fix the EGR you might want to put the new diaphragm in the FPR while you are there. I would stick with the stock type and all will work just great.

When I did my EGR I ended up doing a whole lot more work than just the EGR. On my Corvette the EGR Vacuum Solenoid was not working and I believe this led to the EGR getting carbonized in place. The only sign that something was amiss was when the insulation on the little corrugated pipe on the passengers side of the engine near the firewall started burning off. I was surprised it was getting hot enough to burn the insulation..
In the end the EGR job required a lot more parts, gaskets and TIME than I had expected.

Lots of people don't like the EGR and they try to disable it. You can't just unplug the solenoid to turn off the EGR system. It is built into the code that our Engine use. To remove the EGR you have to get new code without the EGR being in there any more. The C4's run great with their EGR so I wouldn't suggest removing it.

Those FSM's are worth their weight in gold when you need one. Very complete and well written. When you see your engine running on a scanner you will see just how valuable that tool is as well, "live data" is awesome and very handy to have access to.
Start getting some good carbon breaking carburetor cleaner as getting the EGR passages clean can be time consuming. I used carbon buster along with a wire brush to clean out the passages that were blocked with carbon in my engine. The wire brushes were the type you use when cleaning out an engine block. They are long and skinny and worked great on the EGR cleaning. I then used a lot of compressed air to blow out any remaining junk in the lines. It was a project to clean up the passageways.

 

Well, yesterday I got my fuel pressure gauge in and did some testing. When the fuel pump engages pressure was jumping up to 40 psi (not 41-47 as the FSM said it should be, but not concerned about that for now), and then it was rapidly bleeding down within a few seconds as soon as the pump shut off. That indicates a bad fuel pressure regulator is likely as well. So I spent the $90 for that stupid little spring and diaphragm, and opened it up to replace that and the EGR valve at the same time.

A couple of hours later it was all back together to test. This time fuel pressure holds - it does bleed down but over minutes instead of seconds. I was sure it was going to make a big difference, but.....fired it up, warmed it up and drove it around, and it was almost the same. I'm somewhat disappointed. I know I've got some bad-to-marginal vacuum lines, so my next task will be to run new hoses throughout. My wallet is a lot lighter so far with not much to show for it!

 

For some reason the EGR passages were relatively clean. No significant carbon buildup that I could see.

 

On your 91, the MAP sensor is on the side you say you are hearing a vacuum sound, passenger, middle area.
It is on the side of the plenum near your fuel pressure regulator.
Have you run new vacuum lines yet? You mentioned you were going to replace them all in your first post.
Also, with your rough idle, is the idle high or low or variable?
If you have any of those old brittle spaghetti lines, those would go first, in my opinion. They were all cracked up on mine.

 

2 man job but try this. Have hose crimpers ready. Prime the pump and as soon as the crimp guy hears it, crimp off the feed hose and see what happens pressure wise. After that, turn the key off and take it out. Release the crimp. Put it on the return line. Turn the key on and hear it prime. See the pressure. Bottom line, we are seeing which side to look at. If the return line crimp doesn't change anything, you have a problem on the feed side. If the feed side crimp doesn't do anything, it is the side from the crimp onwards.

 

Are we certain he isn't hearing the IAC opening up? It does sound like a vacuum leak which it is.

 

Refresh my memory. Do I have this right?

The Fuel Pressure Regulator diaphragm closes off fuel flow to the injector rail with INCREASED vacuum like at idle. As such, fuel is sent to the Return Line.
When the TB opens, vacuum drops, diaphragm relaxes, and more fuel flows to the injectors.

If I am right, he may have a loose fitting connector to his Fuel Pressure Regulator or the line may be cracked (like mine was). Those big rubber fittings on the end of the spaghetti line stretch out over time. I had to remove mine on my EGR valve because I kept getting a Code 32 when I used it. I removed and went straight with a rubber hose. No more issues.

If his rubber connector is loose, he may be hearing that sucking sound when the vacuum is at highest, at idle.

I am clearly reaching for straws here, but this is a no-cost, easy test. Just use an entirely new vacuum line from the plenum to the FP Regulator.

Now - nobody hammer me for tryin’ to help him…..

 

Hello again Incipheus, et al

You are very luck to have caught the problem EGR early as that affects drive ability more than anything. When mine failed it took many hours of scratching out the carbon in some of the passageways. You live a charmed life my friend.

The EGR Vacuum Solenoid is relatively easy to get compared to 10-15 years ago. I ended up using one made by GM for another vehicle that used the same parts. I was so happy to see them available as I keep my C4 street legal and get it inspected even though it is not required.

You don't have the Cold Start Injector also known as the "9th injector" on your year Corvette do you? I have one on my 1988 C4 and I hate the blooming thing. I have it working now but at one point it was leaking fuel when it shouldn't have.

There is a vacuum connection that disables the cruise control while driving and touching the brake pedal. It is attached down below deep under the dashboard. If and when replacing vacuum hoses be sure to replace these. I am no longer flexible enough to do the brake ones anymore....

A Positive Crankcase Ventilation valve (PCV) have a spring in them and I have hear of those opening and closing.

It might be fun to run your engine after dark with no ambient light and see if there are any Sparks jumping in your engine compartment. I have found more bad spark plug wires at night that way. Some can be corrected by re-running the wire and others replaced if needed.

Got my fingers crossed that your Corvette responds well to your "output of love and care" ($) for it!

 

Replaced most of the vacuum lines, and checked almost all of the others. Still no tangible difference in how it runs. Tested the TPS and MAP sensors today and they appear to be working fine. PCV valve is good. Also replaced the breather hose on the passenger side since it has a crack on it. The mystery continues.

 

Decided to look into exhaust manifold leaks today and discovered that the EGR pipe has a major leak at the top connection with the intake manifold! Used a shop vac output connected to the exhaust pipe and some duct tape to seal it off and it was totally obvious. That is surely the sound I could here from that region of the engine. Now looking at whether simply tightening it solves the issue or if it needs a new gasket.

 

If there is valve overlap, could vacuum at the exhaust be finding a way to the manifold?

 

Sorry, I don't follow. What do you mean by valve overlap?

Gaskets for the EGR pipe are not in stock anywhere local - waiting for them to arrive by mail on Thursday. But I did try to tighten the connection, and while it didn't close the leak it did improve - idle and sound seems better so I'm hopeful this will be the dramatic difference I want!

 

Just wondering if there is no way air can make it to the exhaust where the vacuum is. If the intake and exhaust valves are open simultaneously, wouldn't air be able to go from the throttle body to the exhaust?