Detecting a weak cylinder
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Joined: Apr 2003
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From: Long Island, NY
Detecting a weak cylinder
Hi All,
My 1994 7.3 IDI turbo ( not PSD) seems to miss on one cylinder at idle ( the PO had the idle way up to 900 and i lowered it to around 650). When I rev it up to about 1500 it seems to smooth right out.
I've read in this forum that there are a couple ways to detect a weak cylinder:
1. Warm up the engine, and use an ohmmeter to test each cylinder's glow plug. As the glow plug heats up from proper combustion, the resistance changes. A cylinder that is not firing at idle as strong as it should will have a different resistance level than the others.
2. Crack each injector nut until the one you crack doesn't make a difference in the idle, and that is the one that is probably bad.
I have a question about method #2, isn't diesel fuel going to **** all over the engine when you do this ? or is the proper way to wrap the injector under test with a bunch of absorbent rags ?
Also, how are the 7.3 IDI exhaust valves, is it likely that I may have a burned valve ?
thanks for all the great advice on this forum.
Rick M
My 1994 7.3 IDI turbo ( not PSD) seems to miss on one cylinder at idle ( the PO had the idle way up to 900 and i lowered it to around 650). When I rev it up to about 1500 it seems to smooth right out.
I've read in this forum that there are a couple ways to detect a weak cylinder:
1. Warm up the engine, and use an ohmmeter to test each cylinder's glow plug. As the glow plug heats up from proper combustion, the resistance changes. A cylinder that is not firing at idle as strong as it should will have a different resistance level than the others.
2. Crack each injector nut until the one you crack doesn't make a difference in the idle, and that is the one that is probably bad.
I have a question about method #2, isn't diesel fuel going to **** all over the engine when you do this ? or is the proper way to wrap the injector under test with a bunch of absorbent rags ?
Also, how are the 7.3 IDI exhaust valves, is it likely that I may have a burned valve ?
thanks for all the great advice on this forum.
Rick M
Posting Guru
Joined: Oct 2005
Posts: 1,030
Likes: 0
From: Powell River B.C.
Fuel spilled on method 2 isn,t worth worring about, if you find a bad one, if it clears up with increased RPM my guess would be a sticky injector, go with fuel lube additive- double dose, if that doesn't work move on to a compression test. If the problem goes away at higher RPM I doubt you have a valve or compression problem! LOL
Post Fiend
Joined: Nov 2003
Posts: 21,285
Likes: 15
From: Nutter Fort, WV
Yes, you will lose fuel out of the injector line that is loose, but it will not be a huge amount. A shop rag around the injector will be more than able to catch the fuel.
I would be more inclined to think you have an injector that is going bad. Either there is carbon buildup on the tip and it is not spraying correctly or the injector is leaking fuel while it is supposed to be closed.
You could check the #2 method first, then do a compression test and see if the same cylinder is lower than the rest.
Lower compression in the same cylinder would indicate the injector is not at fault.
Equal compression in that cylinder would point right at the injector being the problem.
I would be more inclined to think you have an injector that is going bad. Either there is carbon buildup on the tip and it is not spraying correctly or the injector is leaking fuel while it is supposed to be closed.
You could check the #2 method first, then do a compression test and see if the same cylinder is lower than the rest.
Lower compression in the same cylinder would indicate the injector is not at fault.
Equal compression in that cylinder would point right at the injector being the problem.
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