Hello all! The vehicle for this problem is a 1991 XJ6 ( XJ40) Sovereign with 111K miles. It started with engine surging on low speeds; about 600-1200 RPM at less than 30 MPH. I replaced the EGR valve, idle control motor, O2 sensor, and temp sensor (all aftermarket parts). I also found one vacuum line failure. Every item replaced improves the situation but does not solve it.
Current symptoms are: All issues present at operating temp or ambient air temp greater than 55 F / RPMs oscillate between 650-950 RPM when in gear and coasting; RPMs oscillate between 900 - 1050 when in neutral and coasting; no oscillation present when at a stop in traffic.
Any thoughts? Thanks!

 

Clean the MAF plug and socket and throw some MAF cleaner through the MAF to see if that will steady things down.

 

Hi, it's not an uncommon problem. I've been battling it for too long myself. Search this forum for threads about it. I had one going and I know there was at least one other one. Good luck.

 

Robman25 & jerry_hoback
Thanks for your guidance! The MAF will be my next step. Someone said an injector seal could be causing a vacuum leak; does this theory hold water?
Thanks again!

 

Not for the idle to go up and down, go for the MAF first.

 

Take out the idle air control valve and spray it with electronic cleaner. Also clean the tip and the area where it seats. They get stuck, causing this symptom.

 

Hi FoMoCo64,

When you replaced the IACV stepper motor, did you remove the valve body and clean all the carbon out of the port where the stepper motor pintle seats? A common problem is that carbon buildup prevents the pintle from closing completely. The large images at Jag-Lovers have still not been transferred over to the new server, but you can see my thumbnails and descriptions here:

IACV Stepper Motor

In addition to Rob's good suggestion about cleaning the MAF and its connector, another task worth doing is to clean all the ground points in the engine bay, especially the ones referenced by the EMS (two on the intake manifold studs, one on the firewall behind the cylinder head) and the engine ground strap.

Have you checked the VCM to see if any DTCs have been flagged?

Cheers,

Don

 

Temporarily disabling the EGR by plugging the hose at the EGR valve with a bolt or something similar can rule that whole system out.

Reset the fueling parameters in the Keep Alive Memory of the ECM by disconnecting both cables on the battery, and shunting the two cables together for 10 seconds (I use a large wrench for this). Reconnect the cables.

Make sure the car is positioned beforehand so it can be driven straight off in forward gear. Start the car from Cold and allow it to idle in Park to Normal Operating Temperature. Do not operate the throttle. Place the transmission in forward gear, drive straight off for at least 50 yards at above 3mph. The idle speed adaptations are relearned.


Nick

 

Thanks all! I cleaned out the MAF with spray cleaner but the problem persisted. I disassembled it next and cleaned with q-tips and electronics cleaner for a deeper clean. The symptoms lessened, but still persist. Operating temperature seems to be when the oscillation really kicks in, but higher ambient temperature makes it almost stall out every time I stop.
The IACM was replaced new, so there was no carbon on the end to worry about. I also took the body off and ran it through a parts cleaner to ensure any carbon or other junk was cleaned out.
I still have a clear VCM; no codes.
I will attempt to clean all grounds and reset the fueling parameters next.
More to follow! Thank you again everyone for the helpful guidance and fresh perspectives to this problem.

 

I cleaned the grounds and reset the fueling parameters, to no avail. Is there any way the MAF could be off, but not throwing a code? The most prevalent symptom is the RPM oscillation gets worse with heat, both off the engine and ambient air, at low speeds (about 30 MPH & slower). There's something about the temperature difference that gets my attention... But I could be stuck on the wrong symptom. Thanks for any input or guidance, folks!

 

Hi FoMoCo64,

While I'm thinking about it, please use your User Control Panel to add your car info to your signature (year, model and engine) so we don't have to scroll back to your first post to be reminded.

We may have mentioned previously that these sorts of problems are often the result of more than one system malfunctioning or operating outside of specification.

Have you connected a vacuum gauge to see if there is any correlation with your surging? You could still have a vacuum leak, perhaps in an out-of-the-way spot such as a stuck-open carbon canister purge valve, or the underside of the intake air elbow which can collapse if the hose clamp is overtightened, creating a leak.

Another possibility is a fuel pressure problem. The XJ40 doesn't have a Schrader valve on the fuel rail, so measuring fuel pressure requires some fittings that aren't in the average fuel pressure test kit. You can check fuel flow rate by disconnecting the supply hose from the firewall end of the fuel rail, directing the end of the hose into a suitable catch container, then either cranking the engine for 15 seconds or jumping the fuel pump relay socket to run the pump for 15 seconds, then measuring the amount of fuel and multiplying by 4 to determine the flow rate per minute. Common issues on XJ40s include failure of the fuel check valve and/or Fuel Pressure Regulator, as well as failure of the fuel pump itself. You can do an easy check on the FPR by disconnecting the vacuum hose, cranking the engine, then checking for any wet fuel at the vacuum hose fitting, which indicates failure of the FPR diaphragm.

Another thing that comes to mind: is there any indication the engine may be running rich? If the Coolant Temperature Sensor sticks at a resistance that tells the Engine Control Module the coolant is cold, the ECM will continue to apply start-up/warm-up fuel enrichment even after the engine has reached full operating temperature. The electrical harness for the CTS is a common failure point, either due to deterioration of the connector or of the insulation on the wiring.

One other thing worth checking, as weird as it may sound, is the transmission fluid. Low fluid has been associated with stalling when slowing to make a turn or stop - I believe because the torque converter does not properly disengage and bogs the engine down.

Please keep us informed.

Cheers,

Don