rough idle

[tech jake]

I just got an 86 Grand national. The car runs great and only has 47,000 miles on it. My question is if its normal for these cars to have a slightly rough idle. With the sequential fuel injection I would expect a smooth idle. The car has no misfires and gets up to speed better than expected, (real fast). It also runs down the highway just fine. I also would like to know the boost psi on the turbo from factory. I'm wondering if my car has been upgraded with a cam and if the boost has been changed sometime in its life. The car sat for 5 years before I bought it and the owner didn't know the history of the car. I crossed the fuel pump relay and ran the tank dry for new gas, I replaced the spark plugs, plug wires, and the dreaded magnavox coil for other reasons - not chasing the rough idle.

[Tom Gallagher]

[img]graemlins/wavey.gif[/img] Hey Jake!

How U' Doin'?

You got a Grand National! Great Find!

If the get has Sat Idle for the lenght on time (5 years) You stated there are Always ISSUES Fuel Systems involving cars that have sat Idle for entended periods with fuel going bad and Lacquering.

Time effects the entire system. The Tank, the Lines, the Fuel Pump, filters etc.

With fuel injection there is a lot of pressure compaired to carboration.

From what You are saying the car reacts well when You put Your foot into it. Apparently when idleing there is some type of restriction or blockage ( when not under Pressure)

Perhaps, and I'm just guessing at this, and taking a shot, that there is a low spot in the fuel line or a bend that is restricted or clogged when under idle, but flows well when under pressure.

I imagine that there is an advanced cam, but I wouldn't thing that it would effect ideling properties

Regards,

Tom Gallagher

[tech jake]

On fuel injected systems the pressure is constant because of the fuel regulator. It shouldn't matter if the car is idling or under full throttle. If there is a restriction it would show up under acceleration and the car accelerates just fine. My thinking is if the duration of the cam is more than stock than you lose compression at slower RPMs because of the overlap between the strokes.

[RonLange]

exactly! i would start by checking the timing, though.