Tech Help. 7.4 won't start on idle

I would check for a vacuum leak where the intake manifold mates up to the heads that will cause problems with the engine running correctly at idle
 
Tom Nyeste said:
Well it isn’t getting any better. The tech got the injectors back. One was diagnosed as not atomizing at all and another was off by 5%. Replaced both and it started and idled,,,,,,,,until he flared it and right back to hunting and surging. Next thing to check is the camshaft, thinks it may be wrong cam in the supplied block. Will keep you updated. I suppose mentioning that I’m going to burn it on a forum is basically wrong.
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Tom,
In your first post you indicated that they had confirmed the cam was the original cam....now the mechanic thinks it is the wrong one?

It is possible that the cam is off a tooth which could give the ECM fits. I keep thinking it is a fuel problem but you have done all the diagnostics to run that down. It behaves as a major vacuum leak.

Whatever it is .....is tied to the rebuild so he needs to pull off the water pump, timing chain cover and see what is going on. A single tooth off could create a retarded condition that the ECM cannot compensate for. It will also provide a chance to recheck that harmonic balancer.
 
Right and your cranking compression will be lower Did you ever mention what those numbers are?
 
PlayDate said:
Tom,
In your first post you indicated that they had confirmed the cam was the original cam....now the mechanic thinks it is the wrong one?

It is possible that the cam is off a tooth which could give the ECM fits. I keep thinking it is a fuel problem but you have done all the diagnostics to run that down. It behaves as a major vacuum leak.

Whatever it is .....is tied to the rebuild so he needs to pull off the water pump, timing chain cover and see what is going on. A single tooth off could create a retarded condition that the ECM cannot compensate for. It will also provide a chance to recheck that harmonic balancer.
Click to expand...
PlayDate said:
Tom,
In your first post you indicated that they had confirmed the cam was the original cam....now the mechanic thinks it is the wrong one?

It is possible that the cam is off a tooth which could give the ECM fits. I keep thinking it is a fuel problem but you have done all the diagnostics to run that down. It behaves as a major vacuum leak.

Whatever it is .....is tied to the rebuild so he needs to pull off the water pump, timing chain cover and see what is going on. A single tooth off could create a retarded condition that the ECM cannot compensate for. It will also provide a chance to recheck that harmonic balancer.
Click to expand...


John was originally told that Marine shop that was doing the work supplied the block/engine to machine shop. Got copy of machine shop work order and found that original engine was not rebuildable so different engine was sent for overhaul. Noticed a cam billed on the work order. Called the machine shop yesterday and they said they thought it was compatible cam but we’re going to call the actual company that supplied the cam to confirm it was the right cam. Your right though and the next step is pulling apart. Thanks for the interest, appreciate the input.
 
Tom Nyeste said:
John was originally told that Marine shop that was doing the work supplied the block/engine to machine shop. Got copy of machine shop work order and found that original engine was not rebuildable so different engine was sent for overhaul. Noticed a cam billed on the work order. Called the machine shop yesterday and they said they thought it was compatible cam but we’re going to call the actual company that supplied the cam to confirm it was the right cam. Your right though and the next step is pulling apart. Thanks for the interest, appreciate the input.
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Latest update, just got off the phone with the engine rebuilder and he advised somehow wrong cam was put in engine. He said it is a milder cam than the recommended one but says they are going to look after installing proper cam. Hopefully that is it. Thx Tom
 
Tom Nyeste said:
Latest update, just got off the phone with the engine rebuilder and he advised somehow wrong cam was put in engine. He said it is a milder cam than the recommended one but says they are going to look after installing proper cam. Hopefully that is it. Thx Tom
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That is odd. Generally a milder cam results in less power......not something that looks like a massive vacuum leak. I'm all for them replacing it and making sure it is installed correctly.

The other thing I would check is what @scoflaw recommended above ....a compression check. The other mechanical issue that could mask as a major vacuum leak is a problem with the valves sealing. Since it starts and idles fine.....then when you advance the throttles it starts hunting fits the symptoms. I know you had them rebuilt but since it isn't fuel or spark.....that leaves us with proper compression. Also it would good to have a baseline for the new block.
 
PlayDate said:
That is odd. Generally a milder cam results in less power......not something that looks like a massive vacuum leak. I'm all for them replacing it and making sure it is installed correctly.

The other thing I would check is what @scoflaw recommended above ....a compression check. The other mechanical issue that could mask as a major vacuum leak is a problem with the valves sealing. Since it starts and idles fine.....then when you advance the throttles it starts hunting fits the symptoms. I know you had them rebuilt but since it isn't fuel or spark.....that leaves us with proper compression. Also it would good to have a baseline for the new block.
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Actually won't start on idle circuit. You have to advance throttle to about 1500-2000 RPM and then it will surge up and down for awhile, then you can pull it back to idle after it has warmed up a bit.
 
You mention a different plenum was tried. Did that include a different throttle body?
 
So if it doesn't start unless you open the t-body that tells me the IAC isn't letting in any air. You replaced it, but maybe the ecm has lost contact with it. Common failure with the Mefi-3. Have you had this motor on a scanner to see what the IAC is doing?
 
scoflaw said:
So if it doesn't start unless you open the t-body that tells me the IAC isn't letting in any air. You replaced it, but maybe the ecm has lost contact with it. Common failure with the Mefi-3. Have you had this motor on a scanner to see what the IAC is doing?
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Tech tells me he has had it on scanner. Ill ask him if he rechecked the IAC. We actually tried another IAC hoping it might have been a defective IAC. No luck. Transferred the ECM from good engine to test but didn't help. Thx Tom
 
scoflaw said:
He should be looking at the scanner while it's hunting, that should show something.
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Determined the engine rebuilder had installed the wrong camshaft. Scheduled to pull engine on Oct.19 and I'll advise if this cures the problem. Thanks for the help.

Tom
 
Problem solved. It was the camshaft causing all the problems. Machine shop advised that it was a 7.4 cam but it was milder and they were sure it wouldn’t cause all these idle and surging problems. They installed the proper camshaft and runs great. Idles, smooth engine, acceleration is terrific. I’m a happy camper. Now to see how much of my bill the machine shop is going to cover.
And thanks for all the input, I appreciate the help.

thx Tom
 
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