Not running very well at or near WOT

[HareRazor]

I have a 91 Ford Ranger with the 3.0 5speed. When I push on the gas pedal near or close to WOT. It starts running poorly. I have also started the truck up and had it run like that until it warms up, then it seems fine. I replaced the fuel filter and I thought that might have made a little difference, but not completely. I think it started rougly around the time that I almost ran out of gas. Thats why I replaced the fuel filter. I have ran a lot of fuel system and fuel injector cleaners through there, but I don't think it has helped. I am considering replacing the fuel pump or atleast checking to see what the filter in the tank looks like. I have a fuel pressure tester, but I'm not sure if it will show roughly the same at idle as it would when driving down the road. Any suggestions?

 

[rustybolt]

I'll take a stab at it... For a fuel injected engine to run properly the engine controller relies on good sensor input in order to properly calculate the amount of fuel to inject and ignition timing. I would go though the sensors one by one and make sure they are functioning properly and that the electrical connection at the connector is in good shape. Older FI engines are vulnerable to degraded electrical connections which could cause all sorts of erratic engine operation. Good luck...

 

[HareRazor]

I have replaced the o2, throttle positioning sensor, and the coolant temp sensor. I also cleaned the MAF sensor. I think the IAC is junk though.

 

[rustybolt]

Forgot to comment on the fuel pressure. The system pressure should be the same either at idle or hwy speeds... There's a pressure regulator on fuel distributor rail that's easy and cheap to replace if the system pressure is not correct.... Just a suggestion...

 

[HareRazor]

I think it smells like the truck is burning a little rich. Fuel pressure is 35lbs at idle and jumps up to 40 when i rev the engine. It is also around 40 when i take the vacuum line off the FPR. I have plugs and a new FPR i am going to change just to eliminate the cheap and easys. Any other suggestions?

 

[Beanmachine7000]

Someone else might know this for sure, but I think at WOT the engine ignores most of the sensors and just dumps in fuel...

 

[rustybolt]

Nothing else comes to mind but it's real important that the fuel pressure is properly controlled or else the gas/air ratio gets hosed up. Sounds like you're on the right track with the FPR. Hope that works out for you....

 

[HareRazor]

Replaced spark plugs, ran compression test. Spark plugs looked like new with very little wear, they did have a light grey color to them whereas the new ones were completely black. Both sets are Autolite with the copper core.
Compression test gave
#1 150
#2 145
#3 145
#4 120
#5 125
#6 150
It isn't always at WOT either. I can be driving down the road and it misses for a second then starts running ok until I stomp on the gas. Normally I can drive around town without it doing it unless I'm taking off. It's time for work, so I'll just replace the FPR tomorrow and see what happens.

 

[betomonroy]

Cylinders #4 and #5 concern me as they are together. Check for differences between 'em spark plugs and the other four.
Also check if you are loosing any fluids (coolant or engine oil).
You may have a cracked head that could be communicating this two cylinders without loosing any fluids. Good luck.

 

[HareRazor]

antifreeze is a little low, doesn't appear to be any water in the oil. I'm going to go do a pressure test on the coolant system and see if it's leaking, or if it's just because i have a hole in the top my reservoir letting it evaporate out. FPR didn't help either.

 

[betomonroy]

If you are loosing your coolant throughout the exhaust you should be able to smell it as the engine is runing.

 

[HareRazor]

I believe the coolant system holds good pressure. It leaked down from 13lbs to 8 lbs in about a half hour. I could hear it leaking a smudge from around the pressure tester itself.