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MAP sensor vacuum tube/ rough running

T

truckeejeeper

NAXJA Forum User
Location
Northern California
So my Jeep has been running pretty rough for over a year now. I have posted on here before about it. It idles ok, but when I start off from a stop, it hesitates, "sputters" a bit, until it seems to catch and accelerates fine. It has seemed to be worse up at altitude (7k ft.)

I have replaced the fuel filter, fuel pressure regulator, and IAC valve. I have cleaned the air filter, throttle body, (and ran Sea Foam through it and the gas). The problem has not gone away.

I read something somewhere about a vacuum line going from the throttle body to the MAP sensor causing this problem. Does anyone have any experiences with this, where to get a replacement, etc. Or any other ideas, as I am stumped at this point.

Thx Y'all-

1991 XJ 4.0HO sport 5speed
 
Never got a response to this. Jeep still running rough. Hesitation/sputtering when accelerating from a stop. But also sometimes when getting on the gas too much. Seems to be a bit worse when warmed up, but is fairly constant. Definitely worse at altitude/in the mtns. Really affecting the driveability/reliabilty. Cleaned throttle, IAT sensor. New fuel filter, IAC, O2 sensor (NGK), battery, fuel pressure regulator. No error codes. Drivng me nuts. any thoughts.

My mechanic has suggested maybe a leaky injector. Really need to tackle this, anyone got ideas?

Thx always

1991 4.0HO 5 speed all stock
 
Does it do the same thing cold and hot? In other words does it run OK for a minute or two before it starts acting up?

A lot of things could cause this, the only plan I have is to check one at a time.

Could be a tiny pin hole in the MAP vacuum line. Could be the vacuum line is plugged into the wrong outlet on the TB. One outlet goes all the way throughto vacuum, the other is a blind hole.

Been my experience when my MAP vacuum was compromised, the motor would run really rich, you could smell the gas in the exhaust anywhere near the exhaust outlet. And I had dark sooty exhaust.
 
It almost sounds like the timing is off. The timing on a 4.0 is dead simple so there is not a lot to go wrong unless the chain has stretched too far. There is a cam position sensor in the distributor, there might be a way to test it.

Replacing the injectors is not a bad idea, good maintenance after 25 years.
But I'd try to replace the ignition coil to see what happens. It's one of those simple things that can screw everything up.


Sent from my iPhone using Tapatalk
 
trippled said:
Just buy some bulk vac hose at the parts store.
Click to expand...

If you do this, make sure you match the inside and outside diameter. I erroneously just used a random piece of hose that seemed to fit, it made a big difference, to the negative. Keep hold of the fittings too, they're hard to find.
 
Try disconnecting one fuel injector wire connector plug wire at a time and see if the problem gets worse, if there is no change you have a single cyl miss fire issue, bad plug wire, spark plug, injector or wiring/connector to the injector. Yo have not mentioned the TPS, a bad TPS is one of the first things I would have checked for.

Does the rough running happen in gear and in park?

The Cat converter could be bad, with a loose honey comb bouncing around randomly blocking exhaust.

HV coil/ICM could be going bad or have bad contacts.

Could easily be a MAP or its vacuum line issue. But the Mao sensors almost never go bad.

My first picks would be the MAP vac line and TPS. Or loose sensor ground...

You have checked fuel pressure at idle and while accelerating the engine right?
 
Wow thanks for all the excellent ideas, hadn't thought of many of them. Def not the Cat Conv., it's new. Problem seems to get worse as the engine warms up yes. It happens at idle and when pulling off from at stop, and sometimes when upshifting (5 speed manual) so I guess one could say at low RPMs. Idles steady never dies, but can tell it's "burble-ey" and will be sputtery at take-off. Once it catches, its got good power. But it's often like"RRRR-rrrr-RRRR-rrrrr-RRRR-rrrr" before I get up to a high enough rpm, if that makes sense. DEF worse at altitude. I'll check the vacuum lines but first I will be checking the TPS. I was sure I had replaced that in the past but going over my records, I see I have not. I'll try that first, check vac lines, and post results…

thx much
 
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I'm seeing about 8 different brands of TPS on amazon. Anyone got a recommendation to the best one?

OEM, Wells, Omix-Ada, Delphi, Standard Motor, Tru-Tech, Airtex, Advantech….

thx yall
 
I had some rough running at low rpms. Found a couple of plugs with the gap way too tight. I was chasing vacuum, map, O2 etc. also; Damn internet (and my assumptions).
 
So- Replaced TPS (mopar). Seems to be running a bit better but still rough/sputtering at low rpms. Most noticable at load and low rpms (like pulling out from an uphill stop.) Of course thats the time when you need the juice the most. Its actually kind of dangerous when pulling into traffic.

Is there a break in period for a TPS? Does it need to "relearn" the engine? I've read conflicting info. Things I'm currently also looking for are exposed wires touching the manifold, and plugged vac lines and their sensors. What are the vac lines/sensors going to the intake manifold other than the IAT. Can these cause these problems?

I also am going to replace my valve cover gasket. It leaks a ton and have heard this may dirty up the CPS. Is this true? Need to replace it anyway. Going to take a look at those 2 grommets on the valve cover at the same time. They wiggle pretty easily by hand. Is this normal.

thx...
 
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Have you done a vacuum test yet? It will tell you if you have a leak and if your cat is clogged, plus many other things. Those two are suspect in this case and should at least be ruled out.
 
UPDATE: First, Cat has a bit of a rattle but it's only about 4 years old. So I decided to replace my plugs, which I hadn't done because it was done only about 10k miles ago along with wires, cap, etc. Found a them pretty fouled and a couple with gaps up to .050. just replaced them all since they are cheap, with the Champion 412's, properly gapped. Also replaced vapor line going to cannister, it was hanging down on the valve cover and melting badly. (don't know if this helped but seemed kinda sketchy) Anyway, plugs coupled with the new TPS and a reset of the PCM (battery pos terminal to ground, etc.) and wow this problem is SOLVED. 4.0 is purring like a kitten and running as smooth as she just came off the showroom floor. I think the TPS helped a bunch, there was corrosion on the leads and it was 25yrs old, then the new plugs and reset and WOW. I should have checked the plugs earlier but they aren't that old I didn't think they would be an issue. Wrong.

Thanks for all the great replies and help. Plus I've learned a ton about all the sensors/vac lines etc. because I've checked everything ha.

THX!!
 
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