87 4.0 AW4 MJ quit running?

Driving down the road about 50 mph thinking "this old truck is running great", so I guess I brought this on myself. From 50 mph it started loosing power so I pulled over and tried to restart it, it cranks strong, has strong spark and even fires on a cylinder or two but will not run. My first thought was CPS but changing that made no difference. In addition to todays CPS I recently changed the TPS to solve a transmission problem " shifted rapidly through to 4th gear by 30MPH". within the last few years New fuel pump and filter and IAC. I've owned this truck for about 10 years and I've never found the fuel pump balast resistor which was another suggestion but I discounted that because when I turn the key I hear the fuel pump run. Any help would be greatly appreciated.

Have someone crank it over while you spray some carb cleaner down the throttle body. If it kicks over, you have a fuel delivery problem.

Check the fuel pressure on the Schrader valve on the fuel rail. If you have pressure, you are probably not getting drive to the fuel injectors. A noid light will verify that.

An outside chance would be a clogged CAT but not very probable since most leak enough to at least allow an idle.

Longshot, fuel filter but the above test should tell you if that is the case.

Thanks for the ideas, I'll check those out.

Ballast resistor.

The white thing.



Just hearing the fuel pump run does not discount the resistor. If you can't find it, someone likely bypassed and removed it, which would still allow it to run fine, the pump would be more audible. Pretty much the only down side.

ParadiseXJ said:

Ballast resistor.

The white thing.



Just hearing the fuel pump run does not discount the resistor. If you can't find it, someone likely bypassed and removed it, which would still allow it to run fine, the pump would be more audible. Pretty much the only down side.

Click to expand...

Yep, never had it on this truck. I'm sure that function is covered by some other little majic box but this one has been to Moab a few times and the Rubicon a few times along with many other trips without that particular piece. I always thought it was odd that my 89 has the resistor but my 87 dosn't.

they were dealer installed on 87s if a customer complained about pump noise.

Otherwise they didn't exist until mid 88 when the factory made the change.

get the fuel pressure gauge form the parts store. You should have high 30's for pressure while cranking.

Definitely spray some starting fluid in the throttle body and see if it runs for a couple seconds.

You'll know at that point whether to chase fuel issues or not.