Hello guys. This is for one vehicle I own. I have a 350 engine. 87 block. 1987 Trans am GTA. The engine was supposedly rebuilt (about 4 years ago) BUT it has a steady rumble/SHAKE at all rpms of operation. I tried to check all basics by: added a AFPR set it at 47 psi. Cleaned out plenum and throttle body and IAC COMPLETELY. New distributer/cap/rotor/plugs/wires. It has stock chip. Headers. No cat. Set timing at 6 degress (timing doesnt affect shake rumble at all). Idle is pretty good except for the rumble/shake. This rumble is present all the time and almost feels like an air rachet rumble at higher rpms. Ive taken it around and people throw ides like harmonic balancer, not balanced, torque converter etc etc. What the hell could it be? Thanks in advance!!

 

F-M,

This is like deja-vu all over again...

I had a similar problem wiht my '86 that started at 18,000 miles. []iNOTHING[/i] could be wrong at 18,000 miles, right?

Since the ignition system is essentially new, you should be able to eliminate that. Check vacuum at idle to determine the reading and how steady it is. A vacuum gauge with a bounce is indicative of a leaky intake valve. Popping in the exhaust could mean a leaky exhaust valve.

Try a cylinder compression or leakage test to verify the mechanical condition of the engine. A leaky valve, worn/sticky ring, mechanical damage to the valve train, or failed gasket can all cause power loss on one cylinder.

You can also do a power balance test by warming the engine, allowing it to idle, then removing the spark plug wires one at time.
NOTE: You're dealing with a 45,000 volt potential, so insulated pliers designed for the purpose are a good idea.

As a plug wire is removed, the idle quality should change noticably. It you remove a spark plug wire and nothing changes, you have a "dead" cylinder. Replace the plug wire and move on to the next one.

The same can be done by removing the electrical connectors from fuel injectors on a TPI/MPFI engine one at a time. A "dead" cylinder will be indicated by little or no change when the corresponding injector is unhooked. Replace the connector and proceed to the next cylinder as above.

I had a similar problem and discovered a plugging injector with a poor spray pattern. One Fedex box to Rich at Cruzin' Performance and an $80.00 bill later, and I was a happy camper. Hard to believe that I could have a few bad injectors at less than 20,000 miles, but it happened.

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Later,
Vader
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"Let the bodies hit the floor!"
Adobe Acrobat Reader

 

F-M,

Another possibility is that if the engine was in fact rebuilt, what camshaft was installed? A cam profile with a lot of overlap will certainly cause a rough, lopey idle. Not too friendly to a TPI with a stock PROM, but it might be another explanation. The operation should get smoother by at least 1,200 RPM, however.

------------------
Later,
Vader
------------------
"Let the bodies hit the floor!"
Adobe Acrobat Reader

 

is that what that popping sound is when people rev their engines up, and let off the gas.. then you hear it pop? i also heard it in the '4thgen mass donut' video on this site when he was around 5k rpms.. i've been wondering what caused that forever.

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• 83 Z28-- not a pretty site to look at, 200k miles and im sure it was treated like a rental car every day of its sad life(before i got it).. sold, thank the lord
• 74 Z28-- 383/400, green on black. pretty clean. FOR SALE!!!

 

Thanks Vader for your reply. I will try it out - the plug wire and injector power balance test. I should also do the compression tests. I am just fustrated at this engine. I am rebuilding another 350 but this one has to last some more and it should at least run smooth, not with a shake/rumble (also, cant sell a engine with that problem easily). Just one question, are the symptoms I am describing just like on your '86 with the injector problem (had)? Thanks!

F-Messenger