: Northstar Shake

07-08-11, 08:23 PM
I have an 87 fiero with a 96-99 northstar in it. Everything was running fine last month, its shaking bad now. Engine mounts look fine, icm is new, coils are new, wires and plugs are new. Pcv valve seems fine. Shakes at idle in drive, park, nuetral and recerse. Kind of a shudder at 55 but that may be irrelevent. Shakes at idle so ive ruled out cv joints.

Any ideas?

07-09-11, 09:54 AM
Probably an ignition or fuel delivery miss - I assume you're running a standalone engine management system, so no diagnostics ???

07-09-11, 10:07 AM
Pcm is from a 95 deville concours. Everything is wired to that ecm. No dianostic retrieval method. Ses light has came on but no way to retrieve codes. Ign system is brand new, from module all the way to the plugs. What should i check next? What in the fuel system should i start with?

07-09-11, 10:30 AM
Do you have a mechanics' stethoscope ?? Check each injector for a steady tick-tick-tick-tick at idle - use a length of rubber hose held close to one ear absent the stethoscope or modify one bought at Walgreen. Plug firing - could be anything from the coil(s), ICM, plugs, wires, CKP's, CMP. Without diagnostics you're flying blind. Check every vacuum connection, as well as the overpressure flapper in the p/s pump end of the intake manifold.

07-09-11, 10:35 AM
What is that over presseure thing at the end of the intake manifold? Do they go bad? How do i tell? I have a medical stethscope, ill try that.

07-09-11, 11:37 AM
Take the chest diaphragm off your 'scope. Attach a proper sized 2-foot piece of rubber vacuum tube from the Help! rack at your local parts store; use that rig to snoop for noises.

Your early intake manifold has a flapper valve in the end opposite the throttle body, designed to prevent a backfire from damaging the manifold itself due to overpressure. Those flappers sometimes develop leaks - a big vacuum leak - which really screws up the idle. If you have that leak, the stethoscope will tell you.

07-14-11, 01:42 PM
I checked the injectors; all of them are steadily ticking. ign coils, ICM, plugs, wires are new and are all connected correctly and tight. How am I suppose to check the flapper with the stethsocope?

Any other sugestions? How often do the camshaft and crankshaft postion sensors go bad?

Thank you.

07-14-11, 02:25 PM
Read My Above Post. I also made a little video that has more info / questions


07-14-11, 07:38 PM
sounds like an intake leak. TB gasket or intake gasket?

Also, throttle position sensor, they go bad.

07-15-11, 10:39 AM
How can I check these? I tried propain enrichment to find leaks; showed nothing. I tried carb cleaner, no change in idle....? I am stumped.

07-15-11, 01:22 PM
i can sorta hear the whine but hard to tell. If its a leak you should be able to find it by spraying. What year is the engine (or what year is the intake from). If its a TPS issue, ive seen a few here that go bad and dont throw codes, hard to pin down. The ends of the ring wear down and they dont read right. I think they are somehow related to idle control motor too. Also check the MAF, there are voltage specs here somewhere...