4.0 only runs at idle at full throttle

M

MJlogan

NAXJA Forum User
Location
Upland, CA
Renix motor
240k mi.

About a month ago the jeep started loosing power when I was just cruising along to the point where I had to hold it wide open to get it off the road in first gear. Then it would not even idle at full throttle. I could hear what sounded like a compression leak in its last minute or so of running. I figure it needs a headgasket or the head is cracked since the motor has about 240k on it.

Get the head back from the machine shop. Machinest said the head was a little warped but not too bad. Slap it back on, hook everything up and it wont fire. Hold it wide open while cranking and I got it to fire but it only runs at about 400rpm at full throttle. As soon as I back off, it dies. Still has the compression leak sound just like when it died before the new headgasket.

Here are my ideas:

fuel pressure regulator
some sort of egr problem
timing off somehow

I'm going to pick up a fuel pressure regulator at the junkyard and clean out the egr next weekend. Anyone expirienced this before or have any ideas?
 
Stop throwing parts at it until you understand the problem. If it only runs at full throttle position, that is most likely because at full throttle the computer defaults from using sensor inputs to using pre-programmed settings. Lack of a spark usually leads me to star at the Crankshaft Postion Sensor, after checking that everything is hooked up. When the CPS goes, it may just die outright or it may start with occasional stalls (usually at low rpm, like idling waiting for a light, the stall just as you touch the gas to go!). Try the unhook then reconnect at the connector at the rear left corner of the engine (behind the throttle body, by the firewall). If it then has spark, you need a new CPS. If it doesnt help, could still be the CPS. Have you checked the cap and rotor, made sure the lead to the coil is on properly, and looked to see that the low voltage connection is on? Some things to start on, anyway. By the way, when the EGR goes out then it will stumble or die at idle, but run OK at higher RPMs- EGR (only used up to 1990) doesn't cause no-spark.
Sorry, misread your post. You are on the right track- check fuel pressure with the vacuum line on and off the regulator at the front end of the fuel rail. Sorry I don't have the numbers on this computer, but they were posted recently. A search for fuel pressure should bring it up. For the EGR, pull the vacuum line that goes from the flying saucer transducer to the valve, and plug the line. If this solves the problem, then the EGR is stuck partially or fully open. You should be able to wiggle the shaft in and out freely.
 
Last edited:
4xBob said:
Stop throwing parts at it until you understand the problem. If it only runs at full throttle position, that is most likely because at full throttle the computer defaults from using sensor inputs to using pre-programmed settings. Lack of a spark usually leads me to star at the Crankshaft Postion Sensor, after checking that everything is hooked up. When the CPS goes, it may just die outright or it may start with occasional stalls (usually at low rpm, like idling waiting for a light, the stall just as you touch the gas to go!). Try the unhook then reconnect at the connector at the rear left corner of the engine (behind the throttle body, by the firewall). If it then has spark, you need a new CPS. If it doesnt help, could still be the CPS. Have you checked the cap and rotor, made sure the lead to the coil is on properly, and looked to see that the low voltage connection is on? Some things to start on, anyway. By the way, when the EGR goes out then it will stumble or die at idle, but run OK at higher RPMs- EGR (only used up to 1990) doesn't cause no-spark.
Sorry, misread your post. You are on the right track- check fuel pressure with the vacuum line on and off the regulator at the front end of the fuel rail. Sorry I don't have the numbers on this computer, but they were posted recently. A search for fuel pressure should bring it up. For the EGR, pull the vacuum line that goes from the flying saucer transducer to the valve, and plug the line. If this solves the problem, then the EGR is stuck partially or fully open. You should be able to wiggle the shaft in and out freely.
Click to expand...


Thanks for the test procedures. I'm leaning towards EGR stuck open because of the leak sound it make when it runs. Every sensor has been fairly recently replaced but, as far as I know, the EGR and the fuel press. reg. havent been touched which makes me suspicious.
 
Sounds like your exhaust may be plugged. Undue the pipe from the header see if it starts, check vacuum?
 
My cat failed with less than 1000 miles on it. It was hit just right by a rock or road debris, cracking the honeycomb stuff inside, which broke up and turned on itself. unbolting the exhaust at the flange takes 10 minutes and wont cost anything - and is a very valid test.
 
Not to doubt you, but are you sure the exhaust is not plugged? The hissing sound before the engine dies is a classic sign! Back pressure trying to get out somewhere. Muffler could have come loose inside blocking the outlet, pipe collapsed inside, or the Eddie Murphy, "Did you fall for the banana in the tailpipe?" :)
 
cal said:
My cat failed with less than 1000 miles on it. It was hit just right by a rock or road debris, cracking the honeycomb stuff inside, which broke up and turned on itself. unbolting the exhaust at the flange takes 10 minutes and wont cost anything - and is a very valid test.
Click to expand...

I'll definitely give it a shot- little more of a hassle than unbolting the cat flange for me cause the exhaust is all one piece. Not as big a hassle as doing the headgasket i guess...

thanks to all for the advice so far
 
I vote for a bad throttle position sensor or connector. Greg
 
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