Possible EGR Problem - Hopefully!!!

[RonnieH]

A couple of months ago, the car started idling rough ('90 Coupe, 32,000 miles). At start-up, idles fine. Soon as it starts to get up to operating temp, rough idling - the car "lopes" with foot on the brake in "D". Getting up to speed an on the highway, it runs great. I took it to a local shop last week and they spent all day trouble shooting. Did a compression check on a couple of cylinders (the EZ ones to get to). One of the cylinders had 110 psi compression when motor was cool, and then dropped to 90 # as it got warm. Let it cool down and right back up to 110-120#. The guy in the shop thinks there is internal damage to the engine and had me convinced. Last fall, I had a water pump problem and did get warm, but not up into the Hot range, so I thought maybe it had done some damage. I beat myself up for a week thinking I had overheated the engine and ruined a motor w/only 30,000 miles. The car has never been "dogged" and always well maintained. Anyway, after having it checked, my only two options were to live with the problem awhile or put in a crate engine.

After getting over the shock and thinking about it, I tried to come up with other possible causes of the problem. I remember from awhile back that a faulty EGR valve can cause rough idling/loping as the car gets up to operating temperature as it can cause the compression to get out of whack.

Just wondering if someone out there may have had similiar problem with an EGR valve previously or if someone has experienced the same problem as me and cured it by fixing the EGR problem. I may be grasping at straws, but need to eliminate all possible causes before going the crate engine route. Other than the idling, car runs great - averages almost 29 mpg if on the interstate and running the speed limit. No matter what, I can live with the problem for awhile.

I was really wondering if there is a quick way to check by blocking off the EGR or bypassing it. I know it can be checked with a vacuum pump and checking the circuitry, but would like to go a quick check before troubleshooting it further.

Thanks for any help/ideas.

[jmccloud]

A rough idle warm (not cold) often points to an O2 sensor but with your great gas mileage, look somewhere else first.
The EGR should not be active at idle. But they do leak and cause vacuum leaks at idle. It's a pain to remove but blocking it off would be a good test. It will throw a light but bring no harm to the engine and you can remove the block (sheet metal, etc.) after the test.
More likely, you have something else that causes a vacuum leak when warmed up. Check all the vacuum hoses, especially to the brake booster and to the cruise/a-c system. With the age of your car, one of these hoses could be cracked and leak when warm. Squirt water or non-flammable liquid along the intake/head interfaces to check for gasket leaks.
Regarding the compression test; sounds like a bunch of hooey. A 9:1 compression engine should read ~135 psi (9X15psi atmospheric) but will often read significantly higher due to carbon build up. Lower readings cold are not necessarily 'normal' but aren't unusual either. I question the shops ability or integrity. No way would I entertain engine replacement at this point.
A pfft-pfft at idle most often results from a leaky exhaust valve. The compression would fall off significantly if a valve is leaking, perhaps into the 20's. You would feel this as a significant miss under acceleration. Do a lot of vacuum leak searching before worrying about valves.
Hope this windy answer helps and good luck.

[Hib Halverson]

If the engine runs ok after a cold start but then, once it warms, it starts to idle roughly, I think it unlikely the engine has mechanical damage (ie: broken ring, burned valve, etc) because you'd notice rough idle all the time, not just after warmed up.

Is the check engine light on or has it been on? If so, what codes are/were set?

A leaking EGR valve can cause poor idle but, rather than buying parts based based on a guess; I'd diagnose the engine, first. You may find diagnostic information about the EGR system on a 90 in the factory service manual.

You need a scan tester for this but, I'd be interested to know if the lack of smooth idle stability begins when the engine controls enable "closed loop". Also, the above suggestion about the O2 sensor is good advice as is the suggestion about trouble shooting the engine for a vac. leak. A bad vac. leak will often set a code for "lean exhaust" but a "not-so-bad" leak may not set a DTC.

[RonnieH]

Thanks for the feedback so far. When the problem first started, I was guessing a vacuum leak somewhere. You guys confirm what I think - if it was internal engine damage, it should run rough while accelarating and at speed. I'm leaning going the route of blocking off the EGR first

Question - is the EGR on the '90 under the plenum? I assume it is at the rear of the motor, but I looked and didn't see it, so assuming the plenum has to come off to access the EGR.

I have not noticed any codes. I tried checking that the other day by jumping out the diagnostic and grnd. terminal on the ALCL, but I did not even get the Code 12 to flash. I tried this on my previous Vette, but was not able to get a Code 12 then either.

I have a Haynes repair manual and I think it does give some infomation on diagnostic testing of the EGR, but first of all I guess I better find it and gain access to it. I will try blocking it off as a quick check and keep my fingers crossed. Just wondering, which way to check it quickly - disconnect the line to the manifold?

[Hib Halverson]

1) If you ground the diagnostic terminal and you do not get code 12, there's a problem. Either the check engine light bulb has been removed (a common "fix" that some do when they are selling a poorly running car to an unsuspecting buyer), the bulb is burned out or the ECM is faulty. In any event, you need the section in the FSM that covers no code 12 when the DLC diagnostic pin is grounded and the key is on and you need to fix that first , then road test for codes before you move on to other problems

2) Use the Haynes manual to line your catbox. If you are going be doing DIY service work on engine controls in your 90, you need useful service data and that means the factory service manual which can be had from Helm, Inc or MidAmerica. Haynes Manuals are not useful because a) they are too condensed and b) they are often contain errors.

3) If you decide to block the EGR as a test (rather than using the FSM's diagnostic procedure) and drive the car that way, understand that no EGR at part throttle will may cause detonation which the EST will attempt to stop with lots of knock retard. All the KR might make the car run poorly and mask other symptoms.

[RonnieH]

1) If you ground the diagnostic terminal and you do not get code 12, there's a problem. Either the check engine light bulb has been removed (a common "fix" that some do when they are selling a poorly running car to an unsuspecting buyer), the bulb is burned out or the ECM is faulty. In any event, you need the section in the FSM that covers no code 12 when the DLC diagnostic pin is grounded and the key is on and you need to fix that first , then road test for codes before you move on to other problems

2) Use the Haynes manual to line your catbox. If you are going be doing DIY service work on engine controls in your 90, you need useful service data and that means the factory service manual which can be had from Helm, Inc or MidAmerica. Haynes Manuals are not useful because a) they are too condensed and b) they are often contain errors.

3) If you decide to block the EGR as a test (rather than using the FSM's diagnostic procedure) and drive the car that way, understand that no EGR at part throttle will may cause detonation which the EST will attempt to stop with lots of knock retard. All the KR might make the car run poorly and mask other symptoms.

If I block off the EGR, it will be just to see if the car idles rough after coming up to temp. I would not attempt to drive it that way. If the idling smooths out after blocking EGR, I would then go into detailed diagnostic troubleshooting.

I agree about the Haynes manual - I have used it as a general reference - I to have noted many errors. In this case, it shows a picture of the EGR where it isn't and doubt if the EGR in their picture even looks the same.

Searching the previous threads on the forum, seems like there are a lot of threads relating to EGR problems.

As for the code, I know the Check Engine light works - when the Change Oil Light came on a few weeks ago, the Check Engine Light came on shortly thereafter and then went out after a couple of minutes. I'm beginning to think it's something I'm doing not able to get at least a Code 12 since I couldn't get it on my other Vette either, but then again inserting a jumper wire between terminal A & B is not rocket science, so I'm not sure why I am not able to check codes.

Would anyone have a FSM and be willing to copy the troubleshooting procedure for no Code 12 and e-mail it to me?

Thanks again for responding.

[Hib Halverson]

Let me make sure I understand your last.

You say the check engine light has been on but even thought it has, you get nothing when your ground the diagnostic pin in the DLC and turn on the ign?

Just in case the DLC is fubar, try connecting the jumper wire from the diagnostic pin to ground.

[RonnieH]

Let me make sure I understand your last.

You say the check engine light has been on but even thought it has, you get nothing when your ground the diagnostic pin in the DLC and turn on the ign?

Just in case the DLC is fubar, try connecting the jumper wire from the diagnostic pin to ground.

The light came on just briefly just after the Change Oil light came on. I just assumed the Check Engine light came on to bring attention to the Change Oil. When starting up, the Check Engine light comes on just briefly as it is supposed to, so I know it works fine.