Missing - gas shortage when at cruise

[lgrant]

92 Laredo 4.0L
I'm experiencing a high speed "stall" at speeds of 50 mph to 75 mph. The vehicle acts like it's momentairly not getting gas. When driving with cruise control engaged, it will just lose power and begin to decelerate and then it will downshift and reaccelerate back to the cruise setting. After the downshift and when accelerating, it seems to run strong and get gas to the injectors as needed.

This happens only occasionally. Has anyone else experienced this?

Thanks, Larry

[rustedwon]

I had this problem once ,but not with my XJ.I had an 87 Ford 2.3 mpi, and the same thing happened to it,driving great then stalling only to reaccelerate.Two scenarios,1-gummed up fuel filter which led to 2-bad oxygen sensor.After that smooth sailing.

[scuba4321]

Mine had a problem like that ,but not when using cruise control, but when at highway speed in o/d with a small load. I checked and tested every sensor and filter......It turned out to be a fouled plug! Moral here check simple things first! Good Luck Igrant

[lgrant]

UPDATE:
Changed fuel filter - the old one did not seem clogged in any way and new one didn't help. Changed plugs - old ones looked pretty good but the gaps had expanded beyond .050. It was time. The plugs looked good enough that I haven't pursued other parts of the ignition system. I may have to come back to the ignition, but for now I'm searching elsewhere. Problem still exists and becoming slightly more frequent.

At this point, I'm guessing it is one or more sensors (including possibility of egr valve). Anyone got a surefire sensor theory? Let me add the following: I bought the vehicle 5 years ago with 114,000. It now has 151,000. Has generally run very good (as most XJs do) with the exception of this problem. One thing to consider in your analysis is that once the engine "catches" again after the driver or the cruise ctrl punches it and it downshifts, the darn thing runs smooth and reaccelerates back to the cruise speed (its not the cruise control since it will repeat the problem without it on). It generally will go through 3 or 4 cycles of the hesitation and reaccelerating before it resumes to normal. Two obvious things come into play with the downshift - 1) the tranny is doing something different and 2) there is a sizeable drop in vaccum. Does either of those items give you any ideas.

[lgrant]

Further update:
Nothing has been done to this vehicle since I have owned it but a new water pump and front brakes and normal lubrication maintenance.

The check engine light does not come on as a result of this miss/momentary shutdown. Engine never dies like described in similar threads.

[JDPascal]

How do you know there is a "sizeable drop in vacuum"?


Have you had the TCM read for codes??

JD

[Dale Aeppli]

Originally Posted by lgrant

Further update:
Nothing has been done to this vehicle since I have owned it but a new water pump and front brakes and normal lubrication maintenance.

The check engine light does not come on as a result of this miss/momentary shutdown. Engine never dies like described in similar threads.

YOU COULD GET A VACUUM GAUGE HOOK IT UP UNDER HOOD AND RUN THE HOSE THRU HOLE IN FIREWALL TEMPERARLY THAT WAY YOU COULD CHECK VACUUM WHILE DRIVING MIGHT HELP PINPOINT TROUBLE. I RUN A VACUUM GAUGE MOUNTED UNDER DASH. HELPS TROUBLESHOOTING WHY ARE PLUGS 50 THS. I HAVE MSD IGNITION THATS WHAT I FOUND RUNS BEST WITH AND MSD COIL DALE
[email protected]
could also be faulty transducer or vehicle speed sensor on transfer case

[lgrant]

Originally Posted by JDPascal

How do you know there is a "sizeable drop in vacuum"?


Have you had the TCM read for codes??

JD

Regarding the vacuum, I'm just going off of how vehicles used to work - you put your foot in it and initially the vacuum drops way off. As the RPMs increase and throttle eases back, the vacuum recovers.

I have not had it read for codes - I just assumed that since there was no "check engine" light, that there would be no codes. Am I wrong in that assumption.

[lgrant]

Originally Posted by Dale Aeppli

YOU COULD GET A VACUUM GAUGE HOOK IT UP UNDER HOOD AND RUN THE HOSE THRU HOLE IN FIREWALL TEMPERARLY THAT WAY YOU COULD CHECK VACUUM WHILE DRIVING MIGHT HELP PINPOINT TROUBLE. I RUN A VACUUM GAUGE MOUNTED UNDER DASH. HELPS TROUBLESHOOTING WHY ARE PLUGS 50 THS. I HAVE MSD IGNITION THATS WHAT I FOUND RUNS BEST WITH AND MSD COIL DALE
[email protected]
could also be faulty transducer or vehicle speed sensor on transfer case

Thanks JD and Dale for your posts. We'll get this figured out (some day).

As I indicated, it ran good with the old plugs and their expanded gaps. But everything is stock and the factory book says .035 gap. If others have had improved performance out of their stock machines with wider gaps, I am entirely willing to follow the performance.

What is a/the transducer?

Is the speed sensor you are referring to the sending unit for the speedometer? How would the speed sensor get involved in the engine performance (remember, this problem occurs with and without the cruise control engaged)?

[Dale Aeppli]

Originally Posted by lgrant

Thanks JD and Dale for your posts. We'll get this figured out (some day).

As I indicated, it ran good with the old plugs and their expanded gaps. But everything is stock and the factory book says .035 gap. If others have had improved performance out of their stock machines with wider gaps, I am entirely willing to follow the performance.

What is a/the transducer?

Is the speed sensor you are referring to the sending unit for the speedometer? How would the speed sensor get involved in the engine performance (remember, this problem occurs with and without the cruise control engaged)?

THE VEHICLE SPEED SENSOR IS MOUNTED ON THE TRANSFER CASE OR THE TAIL SHAFT EXTENSION OF TRANSMISSION. THE TRANSDUCER I'LL HAVE TO LOOK UP IN FSM. MAYBE JD KNOWS THE TROUBLESHOOTING CHART DOESN'T SHOW MUCH DALE

[lgrant]

The problem turned out to be a weak fuel pressure regulator. I took a good look at the vaccum diagram on the firewall and spyed a connection going to the regulator. Putting 2 & 2 together, one can see that at cruise and under high vaccum (and therefore when not a lot of fuel is needed compared to other situations), this relief valve (the regulator) is designed to keep the fuel pressure in the rail lower than for other conditions (high fuel requirements like WOT, etc). The vaccum opens the pressure relief mechanism a little more than if it wasn't present. When you punch it and lose most of the vaccum, the regulator tightens up a little and the pressure in the fuel rail goes up. With my Laredo, when you punched it and it downshifted, it would reaccelerate smoothly because it was now getting the fuel it needed because of the increased pressure.

So far I've just removed the vaccum line and plugged/capped the port. The problem has cleared up but this configuration is probably not the best for the fuel pump - I may have higher fuel pressure in the rail on balance than is desireable. I'll replace the regulator soon.