4.0 liter runs rough after it cools down for 30min

[bigbadbiker]

I just bought this 1994 Mazda B4000 4x4 with 125,000 miles on it. It runs and drives pretty good But it has a weird problem. Aside from a little stumble at idle sometimes. If it is over 70 dagrees when the truck gets to operating temps and then cools down for a half hour or so it runs like crap for a few minutes. If i let it cool down completely it starts and runs just fine. I have replaced the engine coolant temp sensor and the mass air flow sensor. And It has new plugs and wires. Has anyone else had the problem? Any suggestions would be great!!!

 

[MrShorty]

My first thought with a hard warm start/rough running on warm start is a leaking fuel pressure regulator (FPR). When the diaphragm ruptures, it lets unmetered fuel into the intake -- effectively "flooding" the engine until the excess gas has time to dissipate. Next time you shut the engine off, pull the vacuum line to the FPR and see if there's gas in that vacuum line. Any gas= bad FPR.

From there, I'd probably turn to some basic diagnostics: pull codes and check fuel pressure.

 

[mechjames]

There is usually two temp sensors. One for the dash, and one for the computer. If you replaced the one for the dash, the computer is still seeing it through the eye's of its old sensor.

 

[bigbadbiker]

i replaced the sensor for the computer. I also checked the vacume line to the fuel pressure regulator and there was no gas in it.