Mechanics

98 2.4 ROUGH IDLE

1998 Chevrolet Malibu

Approx 99,000 miles.

Car just started rough idle / miss at a stop sign and or in the driveway. Happens when cold or warm. Almost want to stall, if you kick the idle up to 900 or higher, it sounds fine. No OBD codes.

The cylinders are all within 5lbs of each other at 235lbs of compression. Vacuum is below normal at idle. A shop said it was dumping a bit more gas at idle into two cylinders than it should be and vacuum was a little low and it was missfiring on all cylinders at idle. They tested the O2 sensor and opened and closed all sensors with their test machine.

Their recommendation was a full tune up. I had changed air filter, OEM plugs, new coil and sparkplug boots. New egr valve, fuel control valve, idle solenoid,
o2 sensor, gas cap, and radiator cap.

All the plugs looked good, no foul or burning on old plugs.

The car seems to have normal power when driving and doesn't make any abnormal sounds.

I am lost and would appreciate any suggestions. Would a CAT converter being plugged cause rough idle and low vacuum maybe? If so, any test for it/

Thanks for having a service like this.

Rod


Get a pressure gauge and remove oxygen sensor hook it up, a reading of 1.25 and higher will indciate a clogged exhaust system creating the backpressure.

Or remove the exhaust pipe at the exhaust manifold-what's your ears. Note how the engine reacts.

Vacuum Gauge reading: high reading when engine first started, falling back to 1 or zero and gradually increasing to 15 or 17.

Any certain gauge I need to use?

I had taken out the O2 sensor and listened, seemed to still idle rough. Maybe that hole isn't big enough to make a change.

When you put Vacuum Gauge reading info, does that mean it should do this when I test the vacuum?

What about teh MAP or MAF sensor needing cleaned? I don't think it has a MAF though.

Thanks
Rod
Avatar
Ri01rod2
September 22, 2007.



Could have a bad injector or 2. Do all the plugs look even in color? Measure the resistance across all th einjectors. If need be have your local mechanic do an injector waveform on it to look for the miss.

Tiny
Jack42
Sep 23, 2007.
Thanks for the quick reply!

My plugs looked good for being what appeared to be original with 99,000 miles on them.

I do have 4 injectors and fuel rail from another malibu engine of the same year.

What is the resistance supposed to be on the injectors?

I will test that first an dif I find a bad one, change them out for the ones I know is good.

UPDATE: I tested injectors all at 2.4 ohms. I changed them out anyway. Still a bad surge at idle.

I now have a code of P0300 random misfire, I did not have this code before, however the shop that tested the car said I had a random misfire and it did not show up as a code on my scanner and I put over 500 miles on car after they tested it.

Does this P0300 code help/

Tiny
Ri01rod2
Sep 23, 2007.
[quote=" ri01rod2" ]Thanks for the quick reply!

My plugs looked good for being what appeared to be original with 99,000 miles on them.

I do have 4 injectors and fuel rail from another malibu engine of the same year.

What is the resistance supposed to be on the injectors?

I will test that first an dif I find a bad one, change them out for the ones I know is good.

UPDATE: I tested injectors all at 2.4 ohms. I changed them out anyway. Still a bad surge at idle.

I now have a code of P0300 random misfire, I did not have this code before, however the shop that tested the car said I had a random misfire and it did not show up as a code on my scanner and I put over 500 miles on car after they tested it.

Does this P0300 code

Its a random misfire normally caused by vacuum leak and bad gas. If not start by checking the EGR system.

Rasmataz
Sep 23, 2007.
Thanks for the suggestion.

If I pull any vacuum hose of the base of the air intake (carb throttle body or whatever it is called) I get a brief normal idle tehn eratic again.

I have searched high and low and can't find the vacuum leak if there is one, however the vaccum is low. Anny suggestions on locating a leak like this. I am going nuts looking for it.

The shop that did a scope test said he operated the EGR valve and it opened and shut as is was supposed to. I also pulle it off and looked at it, not carbon or anything. Put a new EGR gasket on as well.

I think the vacuum idea sounds correct. Is there a vaccum that goes to the tranny somewhere/

Tiny
Ri01rod2
Sep 23, 2007.
I think the vacuum idea sounds correct. Is there a vaccum that goes to the tranny somewhere/

Not on todays vehicles that's being done by the vehicle speed sensor.

Double check the vacuum hose going to the brake booster also at the PCV valve and anything vacuum operated devices. Like MAF or MAP sensor.

How low is the vacuum?

Rasmataz
Sep 23, 2007.
Vacuum is 11-14lbs eratic 9500-800 rpm, up to 15 at faster idle (900 rpm & Up).
Thanks
Rod

Tiny
Ri01rod2
Sep 23, 2007.
Check the ignition timing for retardation

I don't think you have a vac. Leak your good on that reading- its ok if your down to 3-5inches that low then will be concern.

About the compression pressure ain't that little to high for that engine.

Misfires can be caused by worn or fouled spark plugs, a weak spark (weak coil, bad spark plug wire), loss of compression, vacuum leaks, anything that causes an unusually lean fuel mixture (lean misfire), an EGR valve that is stuck open, dirty fuel injectors, low fuel pressure, or even bad

Rasmataz
Sep 23, 2007.
YOUR POST CUT OFF AFTER COUL DEVEN BE CASUED BY________. WHAT WAS THE REST OF THE POST?

Also, Don't know what compression should be, but it was consistant. I will check timing, good idea. Will follow-up wiht status tommorow evening.

Thanks again,
Rod

Tiny
Ri01rod2
Sep 23, 2007.
Don't know what happened, am having a time opening your posts-what I was trying to tell you is the vac readings seems okay-for 15inch however its flutuating intake leak will take it down to 3 to 5inc level and remain there.

Also triple check the vac. Lines to brake booster/PCV valve.

BTW did you checked the passages on the EGR.

Later

Rasmataz
Sep 24, 2007.
UPDATE: Finally took car to another shop and they had it two days. The conclusion, everything test fine except minimum to no vacuum at idle.

They think it may be a bad valve spring, but they could not guarantee it and it would be spendy to fix.

I now have it back. Does a bad valve spring sound reasonable? They checked absolutely everything.

The car runs pretty good above 1000 rpm.

If it could be a bad spring, what all should be replaced as I get into it at home.

Thanks
Rod

Tiny
Ri01rod2
Oct 8, 2007.
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