RPM spike on 91 Ford Thunderbird SC

Hey everyone. I get an intermittent spike in RPM with a lurch in the engine when I’m holding anywhere from 1500-1700 RPM. I say it’s intermittent because it might happen three or four times over the span of a few seconds, and then nothing for a minute or two. I don’t think it’s the transmission because it doesn’t happen during shifting, and I can duplicate the problem in D, N, and P. And suggestions on what could be causing the problem?

Thanks in advance.

1991 Ford Thunderbird SC
3.8L 6 cyl with supercharger
4 spd automatic transmission
172,000 miles

This happens in neutral too? hmmm … that’s a weird one. Like you say, it’s unlikely be a xmission problem then, if it happens sitting in the driveway in neutral, just reving the engine a bit, and the rpm takes off.

This sort of sounds like the engine is surging. Surging can be caused by an improper air/fuel mixure. The root cause is often an unmetered air leak somewhere, or sometimes it is the fuel pressure regulator or fuel pump. And bad O2 sensors can cause this. The other common cause is when the idle adjustment screw is used to fix another problem, without actuallly fixing whatever is causing the idle to be wrong in the first place. This confuses the engine computer.

Probably the first place I’d check is for any stored codes, current or pending. After that, the vacuum hoses and vacuum devices, for a vacuum leak. After that, probably check the fuel pressure at the injector rail.

A 91 Ford of any type of idle control problem points to defective Idle Air Control valve.

A co-worker with an early 90’s Ford Taurus had their idle air control valve malfunction while pulling into a gas station. Only this resulted in taking out two gas pumps.

On your year of Ford, the Idle Air Control valve would be the first suspect.


The idle air is famous for this. but check the vacuum and the ignition wires. This car does not have a bunch of software. The IAC is just trying to keep the engine on idle. Other issues will cause this problem. Spark, bad injectors, bad fuel mix from a bad o2 sensor are all in the mix IMHO.

Figured it out a while back, but forgot all about this post.

Turns out the crank position sensor wire was vibrating into the serpentine belt. I guess once the belt chewed through the wire cover, it was giving some funny readings and the engine was trying to catch up to where it thought the crankshaft should be. Replaced the wire and zip tied it, no more problems.

A cheap and simple solution

Double check and make sure the crankshaft pulley (harmonic balancer) isn’t separating and causing things to be ever so slightly out of alignment. As I recall failing balancers was a common problem on the SC engines.