Hi all,

Long story short, the idle in my 3.2 xj40 (1992) is fine, but it isn't. It started going up and down every now and then up to the point where I had to drive it with my left foot on the brake to keep the revs up in traffic lights. The most annoying thing is that the car can run ok for a couple of weeks and then randomly do it...it never came together with any engine check light (on that note, when it goes up and down it overfuels like crazy).

In view of this (oh by the way this was with a freshly fully serviced car), I've changed:
- all the usual servicing items (spark plugs, filters, etc)
- fuel pump (it had a fuel pump issue anyway)
- lamda
- water temp sensor
- air temp sensor
- tried a different MAF
- tried a different ECU
- new ICV
- taken the inlet manifold to bits, cleaned the throttle body and adjusted it. The TPS was off (0.4v where it should be 0.6v for a 3.2, this was last night so I am hoping it had to do with this...don't know).

Now for the weirdest: with the car fully warmed up I follow the procedure to adjust the idle, the annoying thing is that with the ICV fully closed and disconnected, I cannot bring the revs below 800rpm, even with the adjuster fully closed.

The obvious would be an air leak, but I've taken all to bits a few times and smoke tested it and there is nothing...I am unsure about how that thermostatic air valve works (the one on the right hand side of the ICV, that connects to the oil vent via a plastic pipe) so it could also be related to that one as it seems to be sucking in quite significantly, but that should all be accounted for.

Any ideas gents? I am really out of options here. It may never do it again, but I still should be able to adjust the idle to 500-600 rpm with the engine warm and the ICV fully closed.

Cheers,

Miq.

 

Hi Miq.,

Just a few ideas come to mind, and please forgive any that are silly. I'm just thinking out loud:

1. Do you have any fault codes stored in the Vehicle Condition Monitor (VCM)?

2. I'm not sure if your U.K. spec car has a carbon canister, but if so, the purge valve could possibly be stuck open, allowing unmetered air into the intake. Also, if you have an EGR valve, that could be stuck open.

3. When you "took the inlet manifold to bits," did you check the torque of the nuts and screws that secure the manifold to the head? I assume your smoke tests would have revealed a leak at the inlet manifold gasket, but since the torque spec for the nuts and screws is so low, they are known to vibrate loose over time.

4. Have you tested the fuel pressure in the rail? It's not easy on an XJ40 since it requires a special adapter for the gauge, but if you're still on your original Fuel Pressure Regulator (FPR), it is probably no longer holding the correct pressure (approx. 40 psi). One easy check you can do is to pull the vacuum hose off of the FPR and check for wet fuel in the hose, which indicates failure of the diaphragm in the FPR. If you don't see any wet fuel, start the engine and double check. Unmetered fuel entering the inlet manifold via the FPR enrichens the fuel mixture, which would tend to elevate your idle speed.

5. On our '93, I once overtightened the hose clamp at the air intake elbow where it meets the accordioned hose that leads to the throttle body, causing the plastic elbow to collapse a little on its underside, creating an air leak that I couldn't see. I would assume your smoke test would have revealed such a leak on your car, but it might be worth double checking the underside of that joint.

6. While you had the TB off for cleaning, did you flush the inside of the TPS with contact cleaner? I don't think a contaminated TPS is usually associated with your symptoms, but it might be worth the trouble just to rule it out. Also be sure the electrical connector for the TPS is free of any oil contamination.

7. Not sure why I thought of it, but what's the condition of your distributor cap and rotor?

8. I assume you installed a new fuel filter with your new pump and other servicing?

9. It's unlikely that your throttle cable is too tight (usually they stretch and have too much slack), but it would at least be worth checking to be sure the butterfly valve fully closes.

10. Do you have any oil leaks from the crankcase? Since the engine breathing system feeds into the air intake, it seems possible that a leak around the oil filler tube or dipstick tube could, theoretically, lead to excessive airflow from the crankcase into the intake manifold.

11. One other thought is the grounds used by the ECM and fuel injection system. The two or three that connect to intake manifold studs, plus the one on the bulkhead/firewall right behind the cylinder head. It would be worth ensuring all of those are wire-brushed clean, flushed with contact cleaner, then snugged up again. In fact, I would clean the grounds on the wings/fenders also while you're at it.

Cheers,

Don

 

Thanks a lot for the advice Don.

No canister or anything fancy on these, so less things to check!

After driving it for about 200 miles the car seems fine now but it may come back at any point...I think that your points 10 and 11 are quite good and they are the only thing on your list that I haven't yet checked so worth spending a few more minutes on it. The most annoying thing is how intermittent the fault is!

Thanks again!

Miq.