• Welcome to the new NAXJA Forum! If your password does not work, please use "Forgot your password?" link on the log-in page. Please feel free to reach out to [email protected] if we can provide any assistance.

Low RPM stumble/poor idle

B

blaisetd

NAXJA Forum User
Location
Westlake, Louisiana (Southwest)
Yes, i have done extensive searching. I'm having low RPM stumble and poor idle (stalls also). I am also hearing what I think is backfire through the intake at certain RPMs. I have a 1990 DD XJ with a 4.0 and AW4. It has a Rusty's intake, MSD ignition box and a flowmaster. The current mileage is 192K. I rebuilt the engine around 176K. With the rebuild i replaced fuel injectors, oil pump hoses, water pump etc. Replaced CPS recently/before current problem started. Since problem started I’ve replaced fuel filter, cap, wires, plugs, battery, adjusted TPS to 82% input/output (was 77%), checked push rods and gave the valve springs the once over and did a compression check. Compression was between 145 and 155lbs on all cylinders. Do good compression test numbers definitely rule out any valve problems? Unplugging EGR vac lines produces no change in idle. Haven't come up with any vac leaks either. Not sure if its related, but the fuel gauge started reading incorrectly around when my problems started. Anyone got any suggestions, I’m about to throw in the towel and bring it to the stealership.
Thanks in advance blaise
 
have u checked your fuel pressure?
 
And fuel filter.

I had a bad plug once (right out of the box) would do the same thing. Because the plugs were new, it was the last thing I checked. Get one of those laser temp gauges and check each exhaust at the head and see if one is colder. (or you can do it with a spray water bottle if you cant find a temp gauge).
 
Hello again. Changed the O2 a few months back. The fuel filter was one of the first things i changed when the problem started. Been reading my Haynes here at work, i'm gonna look more into the EGR and my fuel pressure in the A.M. I'm want to hold off on bringing it to the stealership....but it is frustrating. It will be definately worth it if i find out its the EGR. Does anybody think the fuel gauge issue is related a fuel pump going out? Thanks for the help so far. I will post the solution...if i find it. Thanks for the help. I feel a membership comming on. blaise
 
fuel gauge - u might want to check your ground on your gas tank.

and check your ISC.
 
Update: did EGR solenoid and valve check according to Haynes this morning. The diaphragm on the valve itself moves fine. Did the vacuum checks on the solenoid itself and got all the correct readings. On the last step the Haynes manual states that the EGR valve can still be bad if the solenoid checks ok. WTH?? How is it checked then? Roll the dice and drop $123.99 on a new one? Anyway I noticed also that the stumble only started after she warmed up/not on the initial start up. Jeepdude what is the ISC? Anyone know if the good compression numbers clear me of a valve problem? blaise
 
ISC - is idle air valve near TPS(throttle posision sensor).
 
i checked mine by unscrewing it and pull it out slowly when jeep ruuning, it should pulse out to compensate for u pulling it out and keep it running. You only need to pull it out 1/2" other wise it could come apart. u could even leave the screws in so it does not fall apart. and to put back in just put back in slowly. this ISC runs slow.
 
Update:
Haven't had much time to troubleshoot lately. Waiting for a fuel pressure tester comming in the mail. Checked the IAC, good. Can a bad break booster be causing this? I noticed my my break pedal went to the floor when i was moving cars around the other day..... blaise
 
Do u have ABS brakes, and did u due a vacuum check?
 
blaisetd said:
Update:
Haven't had much time to troubleshoot lately. Waiting for a fuel pressure tester comming in the mail. Checked the IAC, good. Can a bad break booster be causing this? I noticed my my break pedal went to the floor when i was moving cars around the other day..... blaise
Click to expand...
a leaking brake booster can cause the equivalent of a large vaccum leak. Pull the hose going to the brake booster and plug it to see if it makes a difference.
A failed brake booster should not have any impact on the pedal going to the floor. If anything a failed booster would make the pedal a lot harder to push. The brake booster fails "safe" so if it fails on the road you don't loose your brakes.
Of course if you have the Bendix ABS system everything I just typed is not relevant
 
Non ABS truck. I have been looking/listening for vacuum leaks. Not found any...yet. Got my fuel pressure tester today, going to check that and the break booster.... thanks for the help so far. blaise
 
Ok, recap. 1990 4.0 auto non abs. Problem: Bad idle, stumbling and sometime stalling. The bad idle cycles. Meaning it is not constantly bad. Higher RPM seems fine. When blipping the throttle I can hear what sounds like backfiring through the intake. What I’ve done since problem started. New battery, plugs, wires, cap, rotor, fuel filter. Checked compression (~150 lbs), adjusted TPS, checked/cleaned IAC, looked/listened for vacuum leaks, checked/tightened intake/exhaust bolts, checked pressure on fuel rail 31/39lbs, checked EGR valve according to Haynes manual (this test was inconclusive, but everything before the valve itself checked ok i.e. solenoid and vacuum). Pulling vacuum line off brake booster resulted in higher idle and plugging the line didn’t fix idle problem. Prior to problem I’ve replaced O2, and CPS. Does anyone think the EGR is the culprit? Bad gas, water in the fuel???? blaise
BTW just sent my $$ for 2 year membership via paypal
 
Well, posting inspired me to check out those spark plugs. I pulled them out, the Champion Truck Plugs, and replaced them with regular Champions. I noticed that the gap on the old ones was a little small, .30, should be .35. It now idles better, and does not hesitate quite as much with a blip of the throttle. I am going to test drive it in about an hour.
 
Ok when u put new plugs in , u should gap them, remember that these plugs are for many apllications and factory does a average gap.
 
You must log in or register to reply here.

Similar threads

I
2000 Cherokee XJ Misfire Mystery
Replies
12
Views
166
Tim_MN
Tim_MN
N
Severe stumble, misfire and no start issues
2
Replies
27
Views
4K
maxbraketorque
M
5
1991 xj having a spark, misfire issue
Replies
1
Views
2K
RCP Phx
R
J
Cranks, Fires, Dies - 1991 Briarwood Restoration
Replies
17
Views
3K
Jam Master Jay
J
O
2000 XJ Failed Colorado Emissions high CO
2
Replies
27
Views
2K
Ken M
K
Back
Top