Need help diagnosing engine running rough

[Laird Scooby]

Originally Posted by 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?

Hot engine, unplug connector from ignition amplifier module, remove the RSR (or both fuel pump fuses #11 & #13), remove ALL plugs, wedge the accelerator hard down on the floor, screw the tester into #1 and crank the engine on the key for a few seconds until reading stabliises. Note the reading.

Remove tester and press the reset button to zero the reading, move onto the next cylinder, screw it in, crank as above etc. Repeat for all cylinders.

Refit RSR or fuel pump fuses, reconnect ignition amp module, refit plugs and leads and anything else removed, remove wedge from accelerator and ensure it restarts.

The uniformity of the readings is more important than the actual value, if memory serves they should be ~140-160psi with no more than 10% variation between cylinders.

[SalvadorP]

Originally Posted by Laird Scooby

Hot engine, unplug connector from ignition amplifier module, remove the RSR (or both fuel pump fuses #11 & #13), remove ALL plugs, wedge the accelerator hard down on the floor, screw the tester into #1 and crank the engine on the key for a few seconds until reading stabliises. Note the reading.

Remove tester and press the reset button to zero the reading, move onto the next cylinder, screw it in, crank as above etc. Repeat for all cylinders.

Refit RSR or fuel pump fuses, reconnect ignition amp module, refit plugs and leads and anything else removed, remove wedge from accelerator and ensure it restarts.

The uniformity of the readings is more important than the actual value, if memory serves they should be ~140-160psi with no more than 10% variation between cylinders.

Ok. Thx for the corrections/tips.
What do you mean by wedge the accelerator? Can't i just press the pedal?

[Ian21401]

Wedging the throttle pedal fully down with a suitably sized broom handle or similar braced against the driver’s seat back will be the same as pushing the pedal fully down with your foot but will mean that you do not have to get in and out of the car to test each cylinder. Just reach through the open driver’s window to switch the key to crank the engine.
Do not try to short cut or vary Dave’s instructions. Follow them completely. He knows what he is talking about.
I use that method to check my compressions whenever I change my spark plugs.
My compression tester has a flexible hose the end of which screws into the relevant plug hole. Before I bought mine I borrowed a tester which did not screw into the plug hole and had to be held against the plug hole whilst the engine was cranked over. It was not very good and very difficult to use.
Good luck.

[SalvadorP]

Originally Posted by Ian21401

Wedging the throttle pedal fully down with a suitably sized broom handle or similar braced against the driver’s seat back will be the same as pushing the pedal fully down with your foot but will mean that you do not have to get in and out of the car to test each cylinder. Just reach through the open driver’s window to switch the key to crank the engine.
Do not try to short cut or vary Dave’s instructions. Follow them completely. He knows what he is talking about.
I use that method to check my compressions whenever I change my spark plugs.
My compression tester has a flexible hose the end of which screws into the relevant plug hole. Before I bought mine I borrowed a tester which did not screw into the plug hole and had to be held against the plug hole whilst the engine was cranked over. It was not very good and very difficult to use.
Good luck.

Oh, i see. I will just get in and out then.
Yes, i will follow them to a T.
I know the type you mean. It's the ones that have a rubber on the tip of the metal tube right? The one I ordered has a rubber hose too. 2 size adapters. Seems alright. Not the cheapest one on Amazon. Tomorrow will see.

[Laird Scooby]

Originally Posted by SalvadorP

Oh, i see. I will just get in and out then.
Yes, i will follow them to a T.
I know the type you mean. It's the ones that have a rubber on the tip of the metal tube right? The one I ordered has a rubber hose too. 2 size adapters. Seems alright. Not the cheapest one on Amazon. Tomorrow will see.

The slight problem with getting in and out and using your foot to depress the throttle is repeatability. We're all human and prone to errors, you might think you have the pedal all the way down but when you discover an anomaly in your readings have to do the whole test again or wrongly diagnose a non-existent problem.
By wedging the throttle down firmly, it's held in exactly the same place for all 4 cylinders minimising any errors. Also if you have a remote starter switch (they clip on to the starter terminals) you wouldn't even need to move from under the bonnet during the test but that's a (cheap!) luxury few of us have.

[SalvadorP]

Originally Posted by Laird Scooby

The slight problem with getting in and out and using your foot to depress the throttle is repeatability. We're all human and prone to errors, you might think you have the pedal all the way down but when you discover an anomaly in your readings have to do the whole test again or wrongly diagnose a non-existent problem.
By wedging the throttle down firmly, it's held in exactly the same place for all 4 cylinders minimising any errors. Also if you have a remote starter switch (they clip on to the starter terminals) you wouldn't even need to move from under the bonnet during the test but that's a (cheap!) luxury few of us have.

I see what you mean. Well, hopefully I won't find any anomalies in my readings! Fingers crossed. I'm nervous.

[Laird Scooby]

Originally Posted by SalvadorP

I see what you mean. Well, hopefully I won't find any anomalies in my readings! Fingers crossed. I'm nervous.

Don't be nervous, it won't bite you!

I am guessing it's a new procedure for you but if you follow the instructions, you should get the right results. Once you're done you'll wonder what you were worrying about.

With a bit of luck, you should find the compressions are fairly even across the 4 cylinders and if not, will hopefully find something definite that will give a good clue to what the problem may be.

[SalvadorP]

Originally Posted by Laird Scooby

Don't be nervous, it won't bite you!

I am guessing it's a new procedure for you but if you follow the instructions, you should get the right results. Once you're done you'll wonder what you were worrying about.

With a bit of luck, you should find the compressions are fairly even across the 4 cylinders and if not, will hopefully find something definite that will give a good clue to what the problem may be.

It's not about the procedure I'm nervous. I can always pick up the phone and consult the steps you described. Doesn't seem hard.
I'm nervous about the results and the possibility of having an exhaust valve or piston ring issue. Tomorrow i will know.

[Rversteeg]

Also if you have a remote starter switch (they clip on to the starter terminals) you wouldn't even need to move from under the bonnet during the test but that's a (cheap!) luxury few of us have.

At least on the last series 940 (95-98) this was prepared. 12V on connector 17/1 and the starter motor should crank.

Attachment 140086

[SalvadorP]

Great news I think.
The results are the following:
Cyl #1 - 161
Cyl #2 - 160
Cyl #3 - 162
Cyl #4 - 164

Plugs looked alright to me. Correct me if I'm wrong. They had a bit of dark smoke but no sludge, to oil, no humidity.