Engine needs throttle input to stay running when hot

Question

I have a 1994 Ford Ranger 4.0 L 4x4 5-speed.

The truck runs fine when first started up but when it starts to warm up the rpms start to fluctuate and it makes it almost impossible to drive because it’s really jerky and if I let off the gas it wants to die on me.

So I replaced the coil pack and new plug wires thinking they where just old and getting hot but that didn’t fix my problem. I also had to replace my exhaust because it backfired and blew my muffler up so I had to straight pipe it.

Any ideas? Please help. The check engine light comes and goes (mostly stays off though).

Answer 1

The backfiring points to a fuel-mixture problem. Unburnt fuel is getting out with the exhaust and igniting in the muffler, which means you have too much fuel or too little air in your mixture. That's more or less consistent with the issue where it dies at idle but can be kept running if you open the throttle (which will add more air).

Also be aware that a major exhaust modification like switching to straight pipes might cause these type of problems to become even worse than they were. That can throw your exhaust tuning out of whack, or cause the ECU to go into a 'safe' mode if it can't measure the O2, etc.

I agree you should start by having the codes read, as that will tell you more specifically where the problem is.

Answer 2

My 92 Ford Ranger (fuel injected, 4.0L V6) had a similar problem. Some of the symptoms were:

• When I had my foot on throttle, the RPM's would drop, as if I had taken my foot off pedal. RPM's would fall below idle speed, but not low enough to stall.

• Sometimes with foot on throttle the RPM's would quickly fall and then rise again; causing truck to lurch/jerk forward.

• Check engine light periodically going on and off, even if the RPM's weren't dropping.

• Not entirely positive about this, but I remember this tended to happen after engine had warmed up.

• This often happened right after I went over a big bump.

The solution for me was to replace the Mass Airflow Sensor, located between the air filter and intake manifold. This fixed the problem for around a year, but it recently started happening again. Currently I have the MAF unplugged, and the problem has gone away. In the past, whenever I would unplug the MAF, the problem would cease 100%.

A quick look on Rock Auto shows that both the '92 and '94 4.0L V6's take the same model MAF, so I'd try unplugging it.