Need help diagnosing engine running rough

[Laird Scooby]

Originally Posted by SalvadorP

Went back to the car and did the test you said Dave. Unplugged one injector at a time, and the engine worked different every time.
Then i picked a long screwdriver and listened to all injectors and they are all opening at the same regular pace. Does the unplugging test say anything about the condition of the valves?

Probably fair to rule out the injectors then.

You say the engine smooths out at 1500rpm and upwards? What about above 4500rpm, does it remain smooth?

[SalvadorP]

Originally Posted by Laird Scooby

Probably fair to rule out the injectors then.

You say the engine smooths out at 1500rpm and upwards? What about above 4500rpm, does it remain smooth?

As far as I ever noticed, it is only rough at those rpms. I even made a 1000+km in the last few days and the highway ride is a pleasure, only the exhaust that sounds louder. But never noticed misfires or vibration at higher rpms. But normally it doesn't reach those rpms does it? I would have to rev it there on purpose no?

What's special about 4500rpms?

[Laird Scooby]

Originally Posted by SalvadorP

As far as I ever noticed, it is only rough at those rpms. I even made a 1000+km in the last few days and the highway ride is a pleasure, only the exhaust that sounds louder. But never noticed misfires or vibration at higher rpms. But normally it doesn't reach those rpms does it? I would have to rev it there on purpose no?

What's special about 4500rpms?

What about during acceleration?

The coil gives its highest output between 1500rpm and 4500rpm due to it being in the perfect frequency band to be at its most resonant so gives a higher output. If you have a loose HT lead on a plug or an HT lead or plug that is breaking down slightly, it could be causing the misfire until the voltage rises above a certain point, usually in the 1500-4500rpm range.

That said, if you don't ever take it up to 4500rpm that might be the whole problem, it needs an Italian Decoke.

[SalvadorP]

Originally Posted by Laird Scooby

What about during acceleration?

The coil gives its highest output between 1500rpm and 4500rpm due to it being in the perfect frequency band to be at its most resonant so gives a higher output. If you have a loose HT lead on a plug or an HT lead or plug that is breaking down slightly, it could be causing the misfire until the voltage rises above a certain point, usually in the 1500-4500rpm range.

That said, if you don't ever take it up to 4500rpm that might be the whole problem, it needs an Italian Decoke.

It does not vibrate during acceleration. Only when it's "passing" 1500, to the point that sometimes I just give it a bit more gas so it stops. Or give it a bit of gas when stopped at a red light or intersection. If it's a bit above idle or bellow/above 1500, it runs ok.
What's the normal rpms at 130km/h? 3500ish? I normally don't get above that. Should i rev up the engine to 4500 to see how it behaves?

[Laird Scooby]

Originally Posted by SalvadorP

It does not vibrate during acceleration. Only when it's "passing" 1500, to the point that sometimes I just give it a bit more gas so it stops. Or give it a bit of gas when stopped at a red light or intersection. If it's a bit above idle or bellow/above 1500, it runs ok.
What's the normal rpms at 130km/h? 3500ish? I normally don't get above that. Should i rev up the engine to 4500 to see how it behaves?

Not running a manual i don't know what the rpm at 130kmh/80mph would be but i'd guess about 3500rpm in 5th - try changing down to 4th whic would be ~4000rpm at a guess or even 3rd which would bring it up higher.

Am i right in thinking that even during accelerating you don't go above 3500rpm?

[griston64]

Originally Posted by Laird Scooby

Not running a manual i don't know what the rpm at 130kmh/80mph would be but i'd guess about 3500rpm in 5th - try changing down to 4th whic would be ~4000rpm at a guess or even 3rd which would bring it up higher.

Am i right in thinking that even during accelerating you don't go above 3500rpm?

M90 with 3.54 axle is 2750rpm at 70mph

[SalvadorP]

Originally Posted by Laird Scooby

Not running a manual i don't know what the rpm at 130kmh/80mph would be but i'd guess about 3500rpm in 5th - try changing down to 4th whic would be ~4000rpm at a guess or even 3rd which would bring it up higher.

Am i right in thinking that even during accelerating you don't go above 3500rpm?

Not sure. Probably don't. Next time i drive it around I'll take notes and maybe push it a bit. Or just let it heat up and rev it to 4500 to see.

[TonyS9]

I didn't like the look of the distributor, you need to clean it better, no black marks or specs, they will be conductive carbon (although no signs of tracking). Alcohol is no use for doing that sort of cleaning. White spirit is not recommended for painting cleaning, otherwise any residue will eventually evapourate off, its like WD40. Its fine to leave that on as a water dispersant. Intake cleaner might be needed.

You also need to remove and inspect the rotor arm, if it won't come off get a new one and break it off if necessary. Its a service item. Always good to have a spare anyway, but rarely a problem these days.

Not saying that will definately cure your problem, just good servicing practice. Also clean out the black cover, leads and top of the coil until spotless.

Another frequency related problem can be the ignition amp. If you had spark checkers you might be able to see this. Also check the condition of the crank sensor.

[SalvadorP]

Originally Posted by TonyS9

I didn't like the look of the distributor, you need to clean it better, no black marks or specs, they will be conductive carbon (although no signs of tracking). Alcohol is no use for doing that sort of cleaning. White spirit is not recommended for painting cleaning, otherwise any residue will eventually evapourate off, its like WD40. Its fine to leave that on as a water dispersant. Intake cleaner might be needed.

You also need to remove and inspect the rotor arm, if it won't come off get a new one and break it off if necessary. Its a service item. Always good to have a spare anyway, but rarely a problem these days.

Not saying that will definately cure your problem, just good servicing practice. Also clean out the black cover, leads and top of the coil until spotless.

Another frequency related problem can be the ignition amp. If you had spark checkers you might be able to see this. Also check the condition of the crank sensor.

I'll consider what you said. I'll clean distributor a bit better. Gonna also buy a compression tester to check engine compression. Freaking out with the possibility of bad valves.
If it's all clear, i might buy the spark tester, it's not expensive.

[SalvadorP]

Compression tester arrives tomorrow from Amazon. I never did a c. test.
From what I read i plan on:
Doing it on cold engine.
Remove pump fuse.
Remove one plug at a time to test each cylinder.
By cranking engine 5 times.

Any tips or steps i might be missing? What numbers am I looking for on a >500k km 940? Above 120?