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weird missfire/rough idle (RENIX)

P

philip_g

NAXJA Forum User
Location
back in CO
Just this week the XJ has developed a stumble right about at 1100 RPM, if you short shift or take off in first right around 1000-1100 RPM it's stumble. I didn't think it was the TPS since it's at a specific RPM and seems to have nothing to do with throttle position.

it's also been idling rough, it'll idle perfect for a few seconds, then just a little rough for a few, then normal, repeat. it's almost like on cyl will fire OK then mis a few rotations then fire ok. Tomorrow I'll pull the plugs when it's light out and see how they look, the cap and rotor are new, so are the plug wires, the CPS is a year old. Any ideas to check? Sticky injector? Nothing has changed since it started this.
 
What year? If it's Renix, check EGR functions (advanced search for details, I can't remember), gave me fits like that. I ended up disabling mine. If I recall , I plugged the right angle vacuum to the solenoid.
 
cygnus58 said:
What year? If it's Renix, check EGR functions (advanced search for details, I can't remember), gave me fits like that. I ended up disabling mine. If I recall , I plugged the right angle vacuum to the solenoid.
Click to expand...

I think you can just push on it and see if it moves.
that's a good idea, thanks.
 
Mine had the same symptoms, I just pulled the vacume hose off of the manifold and plugged the manifold port. Works great, I just reconnect it when I have an emissions test.
 
87-90 has the EGR, 91+ does not. Add the O2 sensor to the possible issue list, also check the MAP sensor vacuum line for any leaks at the throttle body. Low compression on any cylinder can cause it, but yours sounds like a sticking EGR or EGR electric solenoid valve.

You still tell us what year your jeep is!!!!!!!
 
Ecomike said:
87-90 has the EGR, 91+ does not.

You still tell us what year your jeep is!!!!!!!
Click to expand...

Mike it says Renix in the title and all Renix era 4.0s are the same set-up IIRC.
 
90Blue_XJ said:
Mike it says Renix in the title and all Renix era 4.0s are the same set-up IIRC.
Click to expand...

Uhh, now that you mention it it does, :shocked: may be why I bothered to open the thread to begin with, LOL, must be short term memory loss. Thanks.

There are some subtle minor differences in the Renix years, some wire colors here and there, the fuel pump ballast resistor was added in 88.
89 or 90 got the enviable ABS brake downgrade....always best to be specifc as to the actual year, model and any and all mods on the beast. The less guessing we have to do the better. Also be nice to know if it is manual or auto, that affects the TPS....
 
Of the things you mentioned as different, the only one related to rough idle would be the TPS if auto vs manual tranny. The other things would not alter the rough idle problem. Please correct me if I am wrong. I am your pupil because most everything I know about Renix stuff came from you or 5-90.
 
90Blue_XJ said:
Of the things you mentioned as different, the only one related to rough idle would be the TPS if auto vs manual tranny. The other things would not alter the rough idle problem. Please correct me if I am wrong. I am your pupil because most everything I know about Renix stuff came from you or 5-90.
Click to expand...
Thanks! You are correct, but it's a good habit to get into to post as many details as possible, as the person asking for help rarely knows what might be a relevant detail to solving his problem. If he did, he would probably know the answer, LOL. In your case your are right on (unless I forgot something, which seems to happen more of late, just too much stuff going on in my head these days :confused1 ) those differences would not be involved in his problem. But the fuel pump ballast resistor might be randomly getting ticked off at (loose connection, who knows) at 1100 rpm. A major stretch in this instance I know, but just trying to help posters with questions get in the habit of posting plenty of details instead of "my jeep won't start, what do I do?"! :banghead: LOL.
 
Which one?
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Follow the valve: Valve - tranducer (that round flat thing with hoses attached) - solenoid - Vacume source at manifold.
 
Ecomike said:
87-90 has the EGR, 91+ does not. Add the O2 sensor to the possible issue list, also check the MAP sensor vacuum line for any leaks at the throttle body. Low compression on any cylinder can cause it, but yours sounds like a sticking EGR or EGR electric solenoid valve.

You still tell us what year your jeep is!!!!!!!
Click to expand...
Map line is fine, though compression on #1 isn't great.

it's an 89.
 
philip_g said:
Map line is fine, though compression on #1 isn't great.

it's an 89.
Click to expand...

So how great is number 1's compression?
 
Ecomike said:
So how great is number 1's compression?
Click to expand...


I don't remember exactly but seem to think it was around 90PSI when the other 5 were closer to 105-110PSI. Keep in mind this is at 6,000 feet too....

It's gotten worse, now it's missing pretty bad at all speeds, gears, and throttle positions. this weekend I'm planning to swap the O2 sensor and I might swap the CPS even though it's only a year old. The MAP too I guess. Sure hate to just throw parts at it though.

it seems to be only when it's warm too, or worse when it's warm.
 
Last edited:
Does it cut out at around 1100? Then pick up and ride fine? I heard of someone who had chased electrical components untill he found out the cam had worn.
 
Compression should be 120 to 150 psi, 90 is too low, and may be a sign of problems developing, but is not quite a disaster yet. It is however low enough to maybe cause rough runnning.

You might try changing the oil and using a higher viscsoity oil, and adding some Lucas oil additive and some Restore (TM) oil aditive to the new oil in place of some of the fresh oil, all to get more compression, and add some brown (dry) fiber pellets (radiator and head gasket sealer additive, I use the dry brown 1" flat top, round sided pellets, I forget the brand name) to the engine coolant which will help reseal any head gasket leaks that may be starting up and dropping the compression.


philip_g said:
I don't remember exactly but seem to think it was around 90PSI when the other 5 were closer to 105-110PSI. Keep in mind this is at 6,000 feet too....

It's gotten worse, now it's missing pretty bad at all speeds, gears, and throttle positions. this weekend I'm planning to swap the O2 sensor and I might swap the CPS even though it's only a year old. The MAP too I guess. Sure hate to just throw parts at it though.

it seems to be only when it's warm too, or worse when it's warm.
Click to expand...
 
OverTheHillsATTW said:
Does it cut out at around 1100? Then pick up and ride fine? I heard of someone who had chased electrical components untill he found out the cam had worn.
Click to expand...
it does have a stumble there but now misses at all RPM's and throttle positions.
 
Ecomike said:
Compression should be 120 to 150 psi, 90 is too low, and may be a sign of problems developing, but is not quite a disaster yet. It is however low enough to maybe cause rough runnning.

You might try changing the oil and using a higher viscsoity oil, and adding some Lucas oil additive and some Restore (TM) oil aditive to the new oil in place of some of the fresh oil, all to get more compression, and add some brown (dry) fiber pellets (radiator and head gasket sealer additive, I use the dry brown 1" flat top, round sided pellets, I forget the brand name) to the engine coolant which will help reseal any head gasket leaks that may be starting up and dropping the compression.
Click to expand...

well 80% of 120PSI is 96, and at this altitude about 80% of sea level is all I would expect so it's right in the ballpark.
 
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