My 1988 Jeep Cherokee Laredo keeps backfiring and hesitating. It is the 4 ltr straight 6 engine. I have checked the fuel pump and it looks like I have good fuel pressure no pressure gauge - just checked the valve on the rod with the injectors) I have replaced the spark plugs, plug wires and distributor cap. I have cleaned the sensors and the fuel injectors, but have not replaced those yet. If I am driving under 1800 rmp or over 3500 rpm there is no stutter or back fire - anything in the middle and the Jeep sounds like a juvenile delinquent on the Fourth of July.
Help please.

 

Check plastic line from map sensor to throttle body for cracks or loose at throttle body yet ?

 

This....

 

Thank you for the suggestion, just checked it, and the hose is good.

Any other ideas? Could the computer possibly be the issue?

 

Visit www.cruiser54.com

 

Anything that makes you think it could be the computer? The ECU shouldn't just "go bad"... I mean it's not unheard of but I think there are more likely causes that you could still investigate first.

I'm sure you're doing this already but I'd suggest being structured in your investigation. It's easy to get caught up and just testing stuff out - so just some advice from someone who's been there

Some questions: Have you tested all of the sensors, or just cleaned the connections? Have you checked your grounds? Have you tested fuel pressure under load?

Does the engine backfire always at those RPMs, even when revving in park or does it do it only when under gear?

If under 1800 RPM/ over 3500 RPM, is it related to fuel pressure (and rail dynamics), or vibration causing a sensor to cut out?

This is why I was thinking fuel pressure under load, because if the pump is sporadically dying or just sluggish enough to cause some odd behaviour under load this could be a cause.
This is also why I suggest you test sensors and grounds. Bad sensors can cut out when under vibration, bad connections or grounds can cut out under vibration.
It could also be really any number of things. I'm just spitballing here. Can't say if these are likely causes or not on this engine.

I'm not an expert on an 88; heck I am a newbie on my '91, but you can't hurt building a fault tree or just tracing everything and working through every function one by one. It's a lot of work but it will beat just swapping parts. Trying to swap an ECU seems annoying so I hope that's not it.

Best luck... I hope someone more familiar can give you more firm guidance! EDIT: I see a link to Cruiser54's site above. I agree that's the right spot to look!

 

I was thinking the computer because when the backfiring gets really bad if I turn the Jeep off and on again it improves somewhat. I checked out the website, it is excellent I am going through their steps. When I cleaned the O2 sensor it was foul. Checking the grounds and sensors again today.

 

Yes, that line is good. No cracks and a tight fit.

 

To add, I checked the fuel pressure with an actual gauge and found it was low - but the new fuel pump did not fix the issue. Still going to check all the sensors and grounds.

 

Get Tips 1 through 5 done first.