Hi everyone,

Just finished reading all the threads on this rough idle issue but mine seems to go a little further. I just purchased the car (93 cpe) a few days ago and became aware of the problem the 2nd day I had it.

Cold... it starts and runs well. I can drive it "hot" (those fans need to come on sooner) for 20-30 minutes without problems. After I get where I am going and turn the engine off for 10-20 minutes it's a different story. It usually starts fine but the idle will quickly deteriorate and the it usually stalls when it's put in gear. Sometimes it will drive ok for a minute or two before it gets worse. then.... if it idles it's at like 200-300 rpm and usually the any throttle input is useless. Then it stalls and may restart but with the same low, rough "hunting" idle. Sometimes I am able to limp it home if I am only a couple block away but its surging and stumbling all the way. If the car sits and cools down for 2-4 hours it usually starts and runs fine like nothing happened. Except today. I was stuck on the side of a road for 2 1/2 hours and had to call AAA and have it towed home. (it's kinda like all the plugs are fouled and someone comes and changes them for me every night) Thank you plug fairy!

Other info: All stock (except K&N filter), The check engine light has only come on once and was before any of this started (for me) and it quickly turned itself off. The gas gauge is inaccurate, it runs out of fuel when the gauge says 100 miles left and just below 1/2 tank. Maybe the sending unit & pump is causing all this??? The only thing I have replaced so far is the vacuum check valve that is located on the right side of the intake. It's to have functioning heater vents. This little pos valve is discontinued by GM already. Only place I could find was NAPA, $5.

something tells me this is fuel related?? Weak pump, leaky injectors etc. Seems if it was spark related it would show up when it was cold or even warm too but who knows.

Thanks in advance..... I'm starting to remember why I got rid of my 70 Z28 years ago. Sorry if some of this doesn't make sense... I'm tired, dirty and smell like a poorly running old engine.

Tre

If the check engine light came on it probably left a code.
Get your paperclip and check for codes.

Welcome. Just bought the car? Fuel filter was plugged on mine when I bought it. Good luck.

Check the fuel pressure. If it has been run out of gas, that can/will burn up the pump.

Check your catalytic converter and all of the above.

If it were me, I'd put in a new fuel and air filter if they haven't been recently replaced, and clean the IAC valve. If that doesn't fix it, check the fuel pressure when the issue happens. If you are losing fuel pressure after putting in a new filter, I'd be suspect of the screen on the fuel pump, or the line above the pump, or the pump itself. Could be the regulator but doubtful, they usually don't fail like that.

It could also be leaking injectors loading it up when it sits, if it is, try holding the pedal to the floor then starting it, to clear the flood. I think that is the clear flood procedure, you can search it to be sure since I'm not positive.

Some of the above are good... but lets review from the top.

This is an LT1, not an L98... so I would be shocked if a cat was doing this. You can take a rubber mallet and gently tap the cats to see if the substrate has collapsed inside. That would lead to problems, but at the same time, I would expect you to see external damage to the cats causing said substrate collapse.

Do NOT clean the IAC. It has a special lube on it that will get eaten by cleaners. If anything, I would remove the throttle body, making damned sure the entire assembly was clean. This would require removing the IAC and inspecting it for carbon build up.

You don't mention mileage on the car.

How long was it sitting before you bought it? I wonder if you have a lot of water built up in the tank. Once the fuel starts flowing, the gas/water mix up... but then again, water would sink down and the car should run like crap all the time.

I would replace the fuel filter. It is a pain in the ass, located along the right front frame rail, next to the cat.

I prefer to release the lines at the filter and the return line, release the lines off the injector rails and pull that sub-assembly out and replace the filter off the car. You will spill some fuel... and it will be stinky...

I am also wondering if the ignition control module isn't dying a slow death. Heat will effect both the module AND the harness leading to it. The module is under the coil, right head, behind the PS reservour. Check the connector. If it is cracked, eroded or otherwise just messed up looking, consider replacing it. The next time the car starts to act up, hit the ICM with a shot of cold air (can of dust-off will do it) and see if things start to improve.

Finally, I wonder what condition the Optispark is in... Check the Tech FAQ for some details on the opti.

Also, I have a tech library article on reading codes. http://www.corvette-guru.com/modules/ ... section/item.php?itemid=3

Thanks for all the replies! I bought a fuel filter and fuel pressure gauge yesterday and went and dug out my k&n cleaner and oil. Time to get to work!

Oh... how do I check for a bad cat??

Quote:

Did you check for codes?

Quote:

Not to put too fine a point on it, but please read my post in detail. It has the procedure to test for a bad cat.

This is a potentially complex problem, and reading these threads accurately will save you a lot of time in in the long run. Sorry if I sound like a hard ass, but attention to detail is so very important when troubleshooting these kinds of issues.

Bogus- thanks for all the info. Car has 123,000 miles. It was driven on weekends (or so he said). I see your 1st post was only 2 min before my response... I'm sure I was typing when yours appeared therefore I didn't see it.

Just checked the fuel pressure and here are the results:

Cold:
-Pulled fuse... car would not start
1. Key in run= 38 psi (audible pump)
[1 min later 36 psi]
2. Engine running= 38 psi and held there (ran about 2 min)
3. Shut off engine= 38 psi (but went to 46 psi 5 min later)

2nd attempt:
1. Key in run= 45 psi
[1 min later still at 45 psi]
2. Engine running= 38 psi
[then removed vacuum at regulator & plugged.... = 47 psi]

* Turned off engine and relieved pressure by pressing the release on the gauge. Pressure instantly fell to Zero and almost no gas came out.

Drove car around, heated it up then let it idle. Car was still running fine. Let it sit for about 20 minutes, then restarted. The rough idle, sputtering, surging and popping condition was now there.

I attached the gauge and attempted to relieve pressure. This time gas kept coming out of the gauge pressure release hose. Held it open for about a minute and gas was still coming out. Pressure was @ 10 psi throughout.

I turned the key to the run position but no audible pump sound and the pressure did not change. Stayed at 10 psi.

HOT
1. Key in run= 12 psi and holds(no audible pump)
2. Engine running= 12 psi (only ran for a few seconds then stalled)

Removed gauge and re-installed for another reading. (also removed fuse, turned engine over a few times and re-installed fuse)

1. Key in run= 0 psi (no audible pump)
2. Crank engine= 0 psi when cranking... obviously wouldn't start.

Let it sit 5 minutes.....
1. Run position= 20 psi
2. Engine running= 20-25 psi (bouncing and erratic)
Engine only ran during this stage for about 15 seconds then stalled.

Bad pump? Relay? Regulator?

Now time to check codes.... will a code reader work or do I have to use the paper clip jumper method?

Thanks for everything so far.. sorry for the length of this. Didn't want to leave any of the findings out.

Ah.. I hate it when posts are right on top of each other like that.

wow. I am betting pump.

The best way to test the regulator is to smell the vacuum line. If it is wet or smells STRONG of fuel, it is failing.

The fuel pump circuit has a fail safe - please note this is a one way fail safe - and that is the oil pressure switch. If the engine is producing 4 PSI of oil pressure, it will run the fuel pump if the relay fails. Please note that this will NOT shut the engine off. The idea is to make sure the engine keeps running. If the op drops that low, you will get all sorts of warnings on the IP that you can safely park it. Also, the pump will prime for 2 seconds. Or should... If pressure is ok, you won't hear it as much.

With all that you are showing, I am betting the pump is over heating and dying.

The way the fuel system works is simple, really. There is so much volume of fuel, that there is no way in hell it will all burn. So it flows through the rail and returns to the tank. This keeps the rail cooler and all but stops vapor lock. At the same time, the tank does get warm. On a hot day, don't be surprised if the fuel temp exceeds 120F.

Fuel pumps are easy... but you may want to consider a full on sender at this time, taking into account the fuel gauge error.

To be honest with you when I first read the post I was thinking ICM. But with your tests now, it's pointing to fuel pump as Andy stated. That coupled with the faulty fuel levels is a strong indication. If the car sat for a while, the pump/sending unit could have gotten corroded pretty badly. Could also be just bad after 123,000 miles. Also, the fuel pumps on these cars are cooled by the gas in the tank. Running it low on fuel (less than 1/4 tank) repeatedly can be harmful and shorten the life of the fuel pump.

Luckily the fuel pumps on these cars are surprisingly easy to change (unlike everything else). It comes right out of the top of the bumper area where the fill tube is. Might be a good idea to check the tank for debris while you're in there.

Matthew

New fuel pump & strainer. The tank and sending unit were clean, no signs of rust or corrosion. So far so good!! Have not been able to duplicate the problems stated earlier. Thanks for all the help everyone! Your insight and contributions are very appreciated.

Did you get your fuel gauge problem fixed while you were in there?

Glad to hear you got it resolved!

yay!

UPDATE: The problem has not returned. The added, or I should say "correct" pressure from the new pump showed me the previous owner did not install the fuel filter very well because it started to leak from the outlet fitting. I replaced it with a new one yesterday. (I think I still smell like gas LOL)

The fuel gauge seems to be functioning properly too. (I thought it might) The car wasn't running out of gas, as the prev owner thought, the weak pump was not able to pick up the fuel.

The idle seems a little low to me. The tach shows it's a bit less than 500 RPM when in gear. (AT) Out of gear it only raises to about 500. This when it's warmed up, it's more like 650-700 RPM when it's first started. I checked the plugs... they look to be new and didn't exhibit any signs of overheating, oil or excess fuel.

I did check codes yesterday. That was kinda cool. The only codes that came up were for the courtesy lights (both history and current). No surprise there.... they don't come on at all. The horn doesn't work and the computer didn't sound it either.