I am having a slightly rolling or oscillating idle condition. It's not even enough to show on the tachometer or, frankly, even feel. But is quite obvious to the ear. Maybe 50 rpm.... up-down, up-down, up-down...constantly. It occurs in "P" or in any gear, as soon as the engine warms up, with/without a/c compressor engaged. Experimenting with different idle speeds makes no difference, nor does setting the throttles at .001 or .003" rather than the prescribed .002"

Can't find any vacuum leak.

Years ago I had this problem on my previous V12. Replacing a dead O2 sensor solved the problem. I will check for this again but the sensors are virtually new so I'm not expecting to find a fault....and the system goes into open loop in "P" anyway.

My question is this:

Could weak overrun valves cause this?

Bywater has mentioned that the overrun valves weaken with age and usually benefit from tightening the screws a turn or two....which I plan on doing just on GP.

Or, should I just remove them? Grant, I believe you've removed them, correct? Any down-side to this?

Any thoughts welcome.

Cheers and thanks
Doug

 

Could it still be the lambda control? My 1989 V12 does similar, slight idle variation in 1-2 s cycle when lambda control is active. I can verify this by looking at the diagnostics plug signals with a scope and the sound clearly correlates with the signals. My car stays in closed loop in "P", this changed at some point in time, earlier cars were in open loop in "P".

 

I doubt the O/R valves are the cause Doug. But why not whip out the airfilters and block off their feed just to be sure ? No ideas on the lamda stuff, thankfully nothing of that sort on mine!

 

Doug,

No downside on any I have removed.

Tidier looking manifolds is a plus.

2 less things to think about is also a plus.

I also doubt they are the issue, but tightening them will do no harm.

The RH FPR is eliminated I assume, as that is documented to cause odd idle rev hiccups.

Trimming the fuel pot in the ECU may be needed, obviously at operating temp.

 

Thanks for the help, Gents.

A light bulb went off about trimming the mixture.

I had trimmed the mix right after getting the car running but later swapped ECUs. I can't remember why I swapped the the ECU (first cup of coffee syndrome) ...... nor do I remember setting the mixture.

Cheers
DD

 

AAAAAAH, the joys of maturity and COFFEE.

 

The first step with idle issues should always be cleaning around the throttle plates, on the inner diameter of the body. Tar builds up here, and really messes up the idle.

 

Too true!

Mine are clean as a whistle ...not enough use to dirty 'em since I got the car up and running

After sorting a few more niggles I'll be driving the car full time and I can begin dirty-ing 'em up.

Cheers
DD

 

Tweaking the adjustment "3 clicks rich" seems to have 95% solved the problem....or at least masked it. Oscillation barely perceptible now. I doubt anyone except me would notice it.

My concern (although I'm not exactly wringing my hands, I assure you) is that this takes the adjustment almost fully rich....thus my concern that I might be masking a problem rather than solving it.

Happy for now; may revisit after knocking a few more small items off my to-do list.

Cheers and thanks
Doug

 

Problem NOT solved after all ! Perhaps my suspicions about masking the problem versus solving it were justified?

Or maybe I had two different faults giving a similar symptom?

The general idle quality remains good as I mentioned above BUT there is still a problem.

If I bring the engine speed up to 1100-1200 RPM and hold the throttle steady the oscillation returns but is much more severe....about a 300 rpm swing. Actually, it's less of an "rolling oscillation" sensation and more of a "cut-off" sensation

So, back to the drawing board. But I did have time for a few quick experiments and observtions:

- Vacuum advance disconnected/connected makes no difference

- O2 sensors connected/disconnected makes no difference

- Forcing a bit of enrichment by closing the full load switch makes no difference

- Will not happen when engine is cold


It's seems similar to the classic stuck AAV secnario of rapid rise of engine speed until the fuel cut-off is reached, at which point the speed drops...then increases again, repeat, repeat. Except I don't have an AAV and the problematic RPM seems too low for the fuel cut-off to come into play.

The throttle blades are both set to .002", by the way. And I don't have a RH/inlet fuel pressure regulator.

I'll try posting a video later.

Cheers
DD

 

Here's the video.

Any thoughts?

Cheers
DD


Editied to add:

This occurs in "P" or "N". If I put the transmission gear and hold the car against the brake, it won't happen.

However, I have had it happen a couple times when driving along under light, steady throttle

 

Doug
This sounds to me like an intermittent extra amount of air getting in. HAve you tried blocking off/checking the seal on, etc etc. the extra air solenoid on A bank airbox?
Also, have you tightened up the manifold attachment nuts (all 24 of them) and the throttle body attachment bolts and checked the TB gaskets? Also have you checked/renewed the hoses attaching the balance pipe at the rear of the engine?
Greg

 

I don't have one.


yes


No...but I glad you mentioned it! Worth checking.



Yes, new and tight.

With extra air coming in I would expect a high idle....which I don't have. But I'll be double checking for unwanted air. Sometimes engines don't behave the way we expect them to

I'm also wondering about some sort of A-bank versus B-bank imbalance?

 

Doug
If all that fails to fix it, I think I would do the following as an elimination procedure:

• Remove air filters and airbox detachable bits
• Block off ALL exits from the airbox except the TBs, including the AAV
• Block off the dizzy vent to AAV pipe and block off the AAV to manifold feed (which will mean sacrificing the 90 degree tube probably.
• Remove the rear crosspipe and block off the manifold where it joins the crosspipe
• Block off ALL vac tubes to do with the advance system, and all vac tube spigots both ends of the manifolds including the FPR vac lines
• Block off the crankcase vent at the front of the B bank airbox

I have probably forgotten some, but the idea is to have JUST the intakes, no balancing pipe and nothing else. Warm her up and see if it has gone. If it has, add them back one by one....
Just a final thought, have you tried another ECU, just in case the overrun cutoff is playing you up, but I cannot see how it can without extra air??
Greg

 

I do have an extra ECU and will try it. Much easier place to start!



Well the extra air is coming in as a result of my right foot opening the throttle to 1100 RPM or so !

Cheers and thanks

Doug

 

Update:

I've a couple things to wrap my hands around and run with.

1) Unplugging the TPS makes the symptom go away entirely. This is repeatable; I experimented a dozen times.

2) If I jiggle the TPS wiring to the TPS connected the engine reacts; an unmistakable stumble and roughness



As for #1 above, how does the ECU interpret an unplugged TPS and what response does it give in return? Have I found a problem or masked a problem?

As for #2 above, I isolated the fault to the section of harness coming off the TPS itself. I opened up the harness and could find no fault, visible to the eye or checking with a DVOM while wiggling the wires. I suspect a problem inside the connector body or with the pins of the TPS. I'll do more checking tomorrow. Regardless of #1 this is a fault that requires correction.

Thoughts?

Cheers
DD

 

#1 has the ECU running the fueling purely on the vac signal from the balance pipe. The engine can be accelerated, but slowly, so the MAP has time to adjust. The TPS gives that "squirt" as its opened, thus richening the fuel for "spit back" elimination.
I have had many issues with that 3 leg TPS, so the 2 leg XJ40 unit is used, (adapted as needed) is spring loaded to idle position, so the "lag" built into the 3 leg is not there. Then the fact the 2 leg is so small I reckon is a non-wiper TPS, and is so accurate once set.

That Lucas style rubber plug is also a known hissy item, and the wires do fracture inside the rubber section.

I'll keep thinking.

 

I was thinking about the TPS recently. I want to replace mine with the new version as it’s original to the car and getting on in age now. In the tradition of creeping work scope I started thinking of way it could be improved. Presented in order of most reasonable to questionable.

Install an inline voltage meter for setting/checking. Very accurate (.01v accuracy) lcd meters are available cheaply.

Low resistance potentiometer in-line for setting proper idle voltage without disassembly. Set to 50% when setting up the TPS initially would allow an adjustment up or down.

Full throttle microswitch. Deliver 5v when throttle capstan closes microswitch. Regardless of TPS output (4.xish).

Idle microswitch. Delivers .32v from a dc switching regulator when capstan closes the microswitch. Relay to swap sources. Transition to TPS voltage may be invisible to the ECU or possibly make it freak out.

Obviously a properly functioning and correctly set TPS is just fine on its own and I should leave well enough alone. But there are times that I’m not driving the Jag that need to be filled with something.

 

I've concluded that the problem is in the TPS itself. Something coming loose inside, poor solder joint....something of that nature. I've just ordered a replacement. Will keep everyone posted.

Thanks for the input

Cheers
DD

 

I did realise Doug! It was the oscillation I was referring to. But as you have found a definite fault in the TPS, with luck that may sort it. Odd symptoms though!
Greg