Author Topic: Fluctuating rpms on part throttle.  (Read 2779 times)

Offline sleag40

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Fluctuating rpms on part throttle.
« on: October 13, 2019, 10:54:03 am »
How do!
When the fast idle emissions test was being performed on my bog standard G40, the rpms rose and fell from 2500 to 1500 every second or so. The idle is perfect + it only happens when up to temperature - whilst warming up it doesn't happen. Before I get stuck into diagnosis, has anyone else experienced and resolved these symptoms?

Cheers, Steve

Offline sleag40

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Re: Fluctuating rpms on part throttle.
« Reply #1 on: October 15, 2019, 06:25:36 pm »
Just to add after a quick play with it this afternoon, it seems fine under load,  I'm thinking vac leak somewhere........

Offline sleag40

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Re: Fluctuating rpms on part throttle.
« Reply #2 on: November 03, 2019, 06:34:06 pm »
Vacuum leak not found. Symptoms persist, once the revs reach 3000, the fluctuation stops. Removing the blue temp sensor plug to put it in service mode results in a drop of revs instead of a rise. Loom test next.  :(

Offline Andy

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Re: Fluctuating rpms on part throttle.
« Reply #3 on: November 03, 2019, 06:38:45 pm »
Is your base ignition timing and idle speed set correctly?

Offline sleag40

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Re: Fluctuating rpms on part throttle.
« Reply #4 on: November 03, 2019, 07:00:58 pm »
Indeed - they were set when the engine went back in the car after its paint job (5 years ago) This issue began during the fast idle emissions test a few weeks ago - nothing has been messed with since it was reassembled chief.

Offline PeteG40

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Re: Fluctuating rpms on part throttle.
« Reply #5 on: November 05, 2019, 09:42:12 am »
the co pot values have been known to wander in my experience - might be worth a check.

Offline sleag40

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Re: Fluctuating rpms on part throttle.
« Reply #6 on: November 05, 2019, 06:57:45 pm »
I remembered to take my multimeter with me today!
the co pot values have been known to wander in my experience - might be worth a check.
CO pot/inlet air temp sensor and their wiring check out fine, as does the blue temp sensor. The throttle closed + WOT switches work fine when tested at the throttle body,  but I'm not getting the beep at the ECU plug (6 +11 and 6 + 15) It seems reasonable to assume that either the number 6 wire or the plug pin at either end of it are at fault.
Please note, electrical stuff is not my best subject!

Cheers, Steve

Offline sleag40

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Re: Fluctuating rpms on part throttle.
« Reply #7 on: December 14, 2019, 12:26:04 pm »
After removing the loom, it checks out ok - back to the drawing board!    :(

Offline sleag40

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Re: Fluctuating rpms on part throttle.
« Reply #8 on: December 14, 2019, 04:34:55 pm »
I've run out of stuff to test, whatever it is is getting worse. Does it when cold now as well as at any rpms. I could do with trying another ecu (but don't have a standard one) Long shot, but if anyone has a spare standard ecu I can borrow to rule it out I'd be most grateful.

Offline hayesey

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Re: Fluctuating rpms on part throttle.
« Reply #9 on: December 16, 2019, 04:59:59 pm »
sent you a PM about spare ECU.

But a couple of other things. In your post from 5th Nov, you said that the WOT & idle switches check out at the throttle body connector but not at the actual ECU pins, is that still the case or is it OK now? That suggests a break in the wiring loom (which wouldn't surprise me at all since 30 year old looms, the wires are brittle now).

Then you also say earlier on that the timing is OK because it was checked 5 years ago. I'd check it again if you haven't, that's a loooong time.
« Last Edit: December 16, 2019, 05:04:19 pm by hayesey »

Offline sleag40

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Re: Fluctuating rpms on part throttle.
« Reply #10 on: December 16, 2019, 05:39:23 pm »
Cheers Paul! I'll be in touch about the ECU.
When I first checked the loom, it was still in the car and hence a bit awkward - I removed it at the weekend, and it tested up fine. Was a bit disappointed that I'd not found the problem. Obviously, when I put it back in, the symptoms persisted.
I need to get a timing light on it really - when the rpms fluctuate, I suspect that the timing is retarding and advancing.

Offline hayesey

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Re: Fluctuating rpms on part throttle.
« Reply #11 on: December 17, 2019, 09:20:45 am »
yeah, be worth checking that the base timing is still OK though.

Offline Andy

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Re: Fluctuating rpms on part throttle.
« Reply #12 on: December 19, 2019, 06:30:05 am »
Did you check the separate injector loom? Most of them are properly crusty by now.

Offline sleag40

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Re: Fluctuating rpms on part throttle.
« Reply #13 on: December 20, 2019, 01:49:32 pm »
Did you check the separate injector loom? Most of them are properly crusty by now.
I had to repair it when the engine was reassembled in 2014. Resistance was the square root of diddly when tested recently.

Offline sleag40

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Re: Fluctuating rpms on part throttle.
« Reply #14 on: December 21, 2019, 07:07:02 pm »
Hayesey's spare ecu solved the problem - normal service was restored.
Is it worth trying to get my faulty ecu repaired, or is finding someone with a spare they're prepared to part with a better bet? I don't know if these things are repairable or not.