Help Please 91' 4L Ho

[Dark Horse]

Please Help, I have a 91 MJ 4L HO with engine electrical issues. I will break it down for ease of ruling stuff out.

 

Engine ran okay when purchased 2 months ago went to restore/build up on it.

Tuned up engine

 

New plugs (not platinum, standard plugs)

New Wires

New Fuel filter

New Cap

New Rotor

 

Cut Cat Conv out, new 2 and 3/4"exhaust and muffler (not from jeep), Disconnected battery when I welded on new pipes to existing.

 

Engine now idles fine but when put under load (in gear) backfires through throttle body, pukes oil/gas mixture into air box, plugs black, no power, Checked plugs/wires crossed multiple times, checked throttle position sensor, checked resistance on new wires, checked coil, checked under cap for arcing issues, checked seems like everything. Have multiple errors and most are Voltage to muliple sensors are above/below range. I can give a breakdown if that will help you guys?

 

Could some of you more experienced guys tell me if you have had these issues, and where to look next. Fuel Injectors? Seem okay but all plugs are black on all cylinders. Timing out? How do you get back into time? Coil? could the coil be intermitent? would it cause same issues as an engine out of time? I have read that MJ 4L use the cam sensor for timing and that they "can"t go out of time?"

 

Please Help

[HOrnbrod]

Check for fault codes.

 

http://comancheclub.com/topic/28111-reading-obdi-91-and-92-mj-fault-codes/

[FrankTheDog]

Why did you put such a large exhaust on it?

Unless you did a lot of performance mods there is no need for an exhaust that large.

I don't know if that would cause problems or not though.

[cruiser54]

Just a thought and I know you say the plug wires are in order, but could they all be off in one direction by chance?

[Dark Horse]

The Exhaust was gifted to me out of a newer Chev that owner was disguarding, and I needed one bad. The Codes that came up were 12, 35, 33, 14, 23, 22, 55. Battery Disconnect, Cooling fan runs, No AC in truck, Map Sensor (how do I check and if I disconnect it will die, So is that mean it is working?), Air intake sensor (Cause backfire?), coolant Sensor, End of codes.

 

Keep them coming please?

[HOrnbrod]

That's a lot of sensor faults. Check for the 5VDC MAP supply voltage across the 3-pin MAP connector w. IGN ON. Purple wire is +, light blue/black outer wire is ground.

 

[Dark Horse]

Could the sensor be bad intermittely or is it a case of on/off good/bad? Also what voltage am I looking for (5 volts) on the purple? What is is range? Thanks for the help.

[HOrnbrod]

You are looking for the 5.0 VDC reference voltage generated by the ECU. Tolerance is ± .3 VDC. If the 5.0 VDC supply is out of tolerance, several HO sensors can be affected and throw multiple codes. When you measure, check the + purple wire with the negative meter probe on battery negative battery terminal first, then check again using the ground pin in the MAP connector.

[Dark Horse]

Map Sensor reads 5.02 volts when tested. Does not seem to be the issue.

[HOrnbrod]

Map Sensor reads 5.02 volts when tested. Does not seem to be the issue.

Did you measure at the two outside MAP connector pins with your meter probes?

 

Also have you had your distributor out of the engine since it last ran okay? Do you have a timing light so you can check the timing?

[Dark Horse]

Turns out it was the fuel pump. Pressure guage was 15psi at rail, at filter, and between filter and tank. Took tank out, replaced the pump and vwa-la runs good. Thanks for the help guys.