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Dodge Playing Taps

Q. Thanks for offering the opportunity to have my question addressed.

I have asked this question elsewhere, and did receive what I consider a rather thoughtful answer, but some questions were asked that required follow-up. However, that source closed before I received an answer based on the additional information requested. Perhaps if I copy what has been written to date into this letter, things will be more clear.

Following is what has been written so far, and thanks for any insight you may offer.

Mike

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ORIGINAL INQUIRY
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I have a '93 Dodge Spirit; 2.5 L engine. Two months ago this car developed a knock, an intermittent knock, but somewhat periodic. It does somewhat follow the rpm of the engine, but only to a point, and when you accelerate enough, the knock disappears. The knock is worst at idle speed. It doesn't sound anything like a rod or lifters.

I have taken the car to a mechanic: He removed the engine and tore it down. Gaskets and such were replaced as the engine went back together, but no reason for the knock could be found. The car didn't knock after being placed back in the car and/or when I first went to retrieve it from the garage. However, it ran very badly, which it did not do before I took it to the garage. I did ask what the mechanic thought might have been the cause of the knock, and he said carbon.

I drove the car around the block a couple of times and returned it to the mechanic. It was simply running too badly. I retrieved it about four days later: no knock and running somewhat better, though not as good as when I originally took the car in.

About two days later the knock started again, not as bad as before, but there, and it gets steadily worse the more the car is driven. Everyone seems baffled by what is causing the knock. If you have any ideas, I would appreciate hearing them.

Mike

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THIS IS THE ANSWER I RECEIVED TO THE INQUIRY ABOVE.
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[06-28-2000] you say it does it at idle - can you tell WHERE on the engine it is coming from? Use a mechanics stethoscope to isolate the problem area. How loud is the knocking? Is it like tapping a pencil or hitting a hammer???

Have you checked the oil pressure with a good mechanical gauge? When the mechanic has the pistons out did he look for skirt damage and measure bore roundness? How about wrist pin knocking?

Pinging or knocking from detonation (carbon buildup is one cause) will normally only happen under load so I don't believe that theory.

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THIS IS MY FOLLOW-UP INQUIRY.
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Thanks for you answer.

Yes, the knock seems loudest at the front of the engine (the right side of the car, since this car has one of those east/west placed engines).

Perhaps I should have told you this before--I suppose you can already tell I tend to be a bit long-winded, and I was trying to cut the length of the letter--but when the knock first started it only occurred when the AC was on. Turn the AC off, the knock went away. This led me to believe the cause was the compressor or the compressor clutch. Not so, when I checked those elements, the knock was there all the time; i.em., with the belt off the compressor.

This is where it stands now, after all the work by the mechanic: Turn the AC on, the engine knocks, or worse, anyway.

I have been told since I first wrote to you that a cracked crankshaft can sound just like what I've described. This comes from a mechanic who is now looking at the car. He is 71 years old and said that he has seen this only twice in his life. His theory is that since the knock gets worse when the AC is turned on, and the only thing directly attached to receive an increased load is the harmonic balancer ((I don't think it's really a harmonic balancer on this car; more of just a pulley) and therefore the crankshaft and all that mess). He has already pulled the timing belt cover(s)--the area where the knock is the loudest--and has found nothing.

Could this be right? And how would a cracked crankshaft make such a sound.

Sorry, I didn't answer your other question: The former mechanic said, after further checking, that the wrist pins were OK.

I am really sorry for the length of this letter.

Mike

A. It looks like you have quite a problem here Mike, but I'll see what I can do for you.

Engine noises can be very difficult to track down since the sound can travel through out the block. A rod knock would have a definite "hammering" sound and would change with engine rpm. A valve tap has that "pencil tapping" sound.

As far as the cracked crankshaft being a possibility, well anything is possible. But in this case I don't think that is very likely. Like the old-timer (and I don't mean this disrespectfully) says, it is very, very rare. I have never seen one although I have seen a broken crankshaft. This was on a Mercedes Benz diesel engine and the engine still ran despite the fact the crankshaft was broken in two pieces between number one and two cylinders.

Piston slap is a distinct possibility. GM had that problem in the mid 80's when the wrong pistons were installed in engines manufactured in Mexico. All to often when an engine is rebuilt, the cylinders and pistons are not measured to insure they fit properly. A good clue is to look at the ridge on the top of the cylinder where the top ring stops. If the ridge is very pronounced, that indicates a lot of wear on the cylinder wall and should definitely be measured. If the ridge is there but not very pronounced, then there has been little wear and the old pistons can be reused without a problem. A piston slap noise would tend to be loudest when the engine is cold and disappear as the engine heats up and the piston expands.

Since you don't have a definite pattern here we may have to look elsewhere for the cause. I noticed the first Technician you wrote to seemed to forget that it was intermittent. I have learned over the years not to ignore any possible clues when trying to diagnosis a problem.

I was working at a Chevrolet-Nissan dealer in Wisconsin when we had a 1992 Nissan Stanza come in with, what sounded like, a valve tap. The same symptoms that you described. After the technician checked the engine out, he said the problem was bad lifters in number one and two cylinders. The car was under warranty so the okay was given to replace the lifters.

The new lifters were installed and the car was cranked over without the ignition connected in order to pump them up. The ignition was reconnected and the car started. Then the mechanics nightmare began, the engine had the same exact noise as when it came in. Needless to say, the Technician was not pleased. When I looked at the car, I ran it and listened to the noise. I tried to trace it and as far as I could tell, the noise was in the front of the engine. There was no rhyme or reason to it. Idle speed, with a load or without, there was no discernable pattern to it.

By now I thought I had figured out what the noise was and proceeded to test my theory. I took all the belts off and looked at them. I knew now that if I ran the engine without them, there would be no noise. And guess what? No noise. I put the belts back on one at a time and ran the engine. When I put the alternator belt back on, the noise returned. Bad alternator? Nope, bad alternator belt. This one was easy to spot because there was a rather large piece of it missing, causing the noise.

I would look real close at the belts, in fact, replace ALL the belts before you do anything else. If the new belts do not eliminate the noise then you have to look further into the engine for the source of the noise.

As for the car running badly after the engine was put back together, it sounds like there's a vacuum leak somewhere. Sometimes when the intake manifold is installed, the intake gasket moves leaving a gap and causing the leak. Sometimes it is installed backwards, easy to do if one is in a hurry or not paying close attention, causing a leak. Or a vacuum line may have broken or left disconnected. Squirt a little carb cleaner into the throttle body throat, if the rpm's pick up and the engine smooths out, then you know the cause is a lean fuel mixture.

Let me know how you make out.

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