I'm having one heck of a time figuring out what the cause of my bad idle is. The engine runs really nice above 850 or 900 RPMs, but below that it stumbles and surges and is generally terrible.

I have no check engine light or VCM codes.

It seems to not happen as much when the car is stone cold, but when it's nice and warm at operating temp, it always happens.

Spark plugs, wires and distributor cap and rotor are new. New air cleaner. All air intake elbows and hoses and whatnot are tight and secure.

It started happening about 2 weeks ago, the day after coming back from a 250 mile road trip. I drove the trip with the check engine light on, due to a bad EGR valve. When I got back, I replaced it with a new OEM unit. I disconnected the battery to reset the light. It idles like crap ever since.

Thinking it might be caused from the EGR valve, I ordered a new gasket and replaced that. I tested the EGR vacuum solenoid. Works as it should, not allowing vacuum to the valve at idle.

I replaced the cam cover gaskets today thinking it may be from the slight amount of oil in the spark plug wells. No change there. I pulled the new Champion plugs and they look good with their 3k miles of use.

I bought a new fuel filter from the dealer but haven't installed it yet. I'm not sure where to look next... Any ideas?

Its most noticeable when I'm sitting at a red light at night, in gear, with the lights, fans, and A/C on. I can actually feel my body shaking from the rough idle. If I just tap the gas a little to get the engine above 850 or 900 RPMs, it smooths right out.

Thanks,
Nick

 

Nick, the usual suggestions for this symptom is a dirty throttle body or maladjusted throttle cable -or- bad connections at MAF or TPS or coil, dirty/plugged breather/egr hoses or port -or- incorrectly set stepper motor and of course a vacuum leak.

 

Thanks Lawrence. I did clean what I could inside the throttle body by removing the intake bellows and getting in there with a rag and carburetor cleaner. I haven't actually removed the throttle body to do an in-depth cleaning, though. I haven't measured the throttle butterfly clearance yet.

I adjusted the idle air control valve per the shop manual, using the ignition on, then off, then unplug, wait, plug back in x 3 procedure, or whatever it was. I found the base idle to be set a little higher than spec. I dropped it down. I think it made the problem worse.

Thanks,
Nick

 

Ten to one it's an electrical connection problem Nick. These cars are electrical-intensive and bad connections cause all kinds of issues. As the car bogs when the there's a high drain (A/c, lights etc on) it seems to me to point to loose or dirty connections prob - that's just my guess. Hope you get it sorted out soon!

 

I'll inspect and clean the TPS, MAF, IACV and ICM connectors and harnesses.

I think I'll also swap the old EGR valve back in, and see if it changes the idle. The new one might be defective, allowing a small amount of EGR gas into the intake a idle.

On the diagnostic symptom chart, it lists EGR System as number 1 for rough idle. IACV Circuit and Setting for number 2, Injectors number 3, MAF Sensor and Circuit number 4, and Ignition Secondary Circuit number five.

I'll give it a go this morning.

Thanks,
Nick

 

I picked up some contact cleaner, cleaned all the connections I could find under the hood, including all the relays and their bases.

I swapped the old (original) EGR valve in, which has a vacuum diaphragm leak, disconnected and reconnected the battery, let the engine idle to warm and drove it a short distance. I then performed the base idle adjustment procedure again, and brought it down even further to within spec.

The car idles very nicely now. The fault seems to be the new EGR valve.

It was a Jaguar OEM EBC2737 EGR Valve. I noticed the vacuum hose connection is a few degrees off from the original, but they look identical physically. Could it be that the new valve is sticking open very slightly? Replacing the gasket didn't change anything.

Thanks,
Nick