I am tackling a distinct engine stumbling issue on my 1990 coupe with Marelli ignition system for quite a while and need some assistance: Starting the engine from cold, the rev counter initially settles at around 1000 rpm and then begins to jump back and forth (quickly) between almost 0 and 1000. No difference noted when in gear or in “P”. Car accelerates normal but when taking the foot off the gas pedal (“sailing”), one can feel some stumbling. The effect almost vanishes with a hot engine. Car starts fine in hot condition, but the stumbling is still present, although much weaker.
Checking at the tailpipes, both engine banks are affected, so the root cause has to be in the common, not bank specific ignition circuit (given the quick oscillation I think this is rather ignition related and not related to fuel delivery. Comments welcome, of course).
I checked all connectors, fuses and wiring but was so far not successful in identifying the culprit. Could it be one of the crank sensors? Any help is greatly appreciated.
Short answer – unlikely.
Bad CPS is better known trouble-maker when hot, not when cold.
But, CPS is one of the very few ignition components that will affect BOTH banks
The stumble can be wrong A/F mixture. Have you checked your throttle bodies and have you verified that the tube connecting the intake to the ECU in the back (rear fender) is intact?
Good luck,
Steve
PS Tach signal is taken from one of the coils btw.
On a couple of occasions I have had front crank sensor issues which manifested as a very brief stumble and the tach drops briefly to zero. I understand that it would be unusual for a crank sensor to get better as things warm up, but at a minimum I would unplug both sensors (front and rear,) give the plugs a good shot of contact cleaner, and re-connect / properly seat. I’d also check the famous white (I think?) wire connector that is on the left (from drivers viewpoint) fuel rail in the harness. This is where one of the sensors (front, I think) connects to the main harness. It is a small plug that is a little sketchy. It does not take much in the way of a poor connection for one of the sensors to get sketchy.
Regards
Bob
thanks for that. I already cleaned the contacts for both sensors and even installed new connectors. I also checked that flimsy white wire and the connector.
I also checked on the fuel pressure regulator as suggested by Baxtor: No effect.
Meanwhile, I wiggled at the coil connector for the “A” bank and had a strong stumbling reaction. I cleaned the connector’s contacts and ran the engine. Much better, but the stumbling is not completely gone (the engine also reached operating temperature where the stumbling is not that prominent anymore).
Based on that my new theory is that the “A” bank coil has an intermittent short in it’s primary coil which also has an impact on the “B” bank coil resulting in that stumble affecting both banks.
Any thoughts on that?
Hello,
Is there any power loss or loss of accel? I had a similar prob (on 2 engines) and it was partially blocked injectors. I gave it a strong dose of injector cleaner and a bit of a blast and the “flutter” at idle was much improved altho not eliminated. If it comes back i’ll get all of the injectors sonic cleaned and take it from there. Charlie.
No loss of acceleration and no power loss (one bank?)…
I had the injectors checked and cleaned a while ago and I occasionally used injector cleaner. Because this stumble or “flutter” as you call it happens so quickly and violently I strongly suspect this to be an electrical issue and not fuel related. BTW I installed a new engine harness as well…
I also replaced the flywheel sensor without success.
I meanwhile replaced one ignition coil (the one that feed the tach) - no change. So I am going to use the leftover coil and replace the other coil with that although I am not too certain that this is the culprit. Have you had any experience with a melted catalyst and its effect on fueling (via the O2 sensors)?
I will also re-gap the the throttle butterflies and will bridge the idle switch when it happens again.
Please note: Judging from the exhaust it affects both engine banks
The A and B bank coils are totally separate one can not affect the other, they are isolated by the ignition modules. The only common connection is the + VE (12volts). Have you checked the wiring for damaged/crusty insulation.
Also the white wire that went to the ignition module in the Lucas cars is joined in the engine bay around the centre of the B bank intake manifold, this wire now goes from the Marelli to the Lucas if this is intermittent the FI ECU will not fire the injectors without this pulse.
Hi Stephan, Given your description its strange that its significantly better when the engine is hot. Dont want to speculate further as you can easliy end up barking up the wrong tree. I know you will have done this so i’m not going to ask you if you have checked the plugs/leads/ insulation…
Charlie thanks: Yes - leads, plugs, insulation are all new (less than 5000 miles).
It seems to me that at times the idle is higher when the engine is running smooth (on all cylinders?). What if the coolant sensor for the EFI is bad, i.e. has intermittent shorts or wrong resistance? The engine stalls when the sensor is disconnected…
I know both banks are independent but I figured that a short in one coil could have an effect on the other side/coil too.
I checked that flimsy white wire and the connector. But I will run a new wire from the engine to the EFI ECU just to make sure.
The wiring for ignition and fuel injection is good - I installed a new harness.
I use NGK plugs with 0.035 gap.
Warren: Good idea with the resistor pack: I did take them out and cleaned the contacts a while ago, I certainly should do it again. But I am not too hopeful that this resolves the issue.
Just for clarification: When the idea drops it feels as if the engine stops for a fraction of a second, rather than a “wandering” of the tach needle.
So I figure it to be a) electrical in nature and b)originating from either voltage supply (incl. grounding) or sensors.
Any opinion on the O2 sensor?
My car is now in winter storage and I will address the issue again in Spring. Any further suggestion is highly appreciated.
The Driver’s Handbook" on page 234 for my wife’s 1990 XJ-S convertible
(5.3L V12 w/ Marelli ignition ) specify NGK BR7EFS plugs gapped to 0.025
inches The earlier cars used Champion N10Y gapped to 0.035 inchest in
North America or 0.025 inches in places except North America. I wonder if
your BR7EF plugs or the 0.035 inch gap you set are contributing to the
stumble problem?
To recap the discussion, BR7EF and BR7EFS are the same plug, sans a small difference in the shape of the electrode.
And apparently, 0.025" is the sweet spot for the gap for all v12.