Mechanics

SURGING AND BACKFIRING

1998 Jeep Cherokee

I have a 1998 Jeep Cherokee with the 4.0L I6. It had a problem where intermittantly it surged and sometimes backfired on acceleration. The more the throttle was depressed the worse the symptom became.

When it first started doing this I discovered that the fuel pressure was sometimes as low as 20 PSI. I changed the fuel pump/filter assembly and the problem disappeared for about 2K miles.

It has now come back but only on moderate throttle. I have checked the fuel pressure at the fuel rail and it appears to always be approximately 50 PSI.

This time the symptom is slightly different. If full (or almost full) throttle is applied during acceleration it accelerates properly and will then continue to run fine under all conditions until the next time the engine idles for an extended period (like at a stop light) or ran at very low throttle for a while like when in 4WD on level but rough surfaces such as a field or if the engine has been turned off.

It appeared that maybe the IAC solenoid was sticking open. I cleaned the solenoid and the the IA port. Again the problem went away for a few days then returned. So I changed the IAC solenoid. This did not change the symptom.

So, as it stands, I have a new fuel pump and fuel filter, a new IAC solenoid and a new rear oxygen sensor (changed for another reason). I still have the problem.

I had the codes read and they didn't show anything.

Any thoughts?
Avatar
Yeloduster
September 5, 2007.



The surging immediately made me think of fuel pump, but you've already addressed that. Ignition timing could also cause your symptoms, but if you have the Inline6, no timing adjustments required. So, the first thing to look at that would cause BOTH problems is a VACUUM Leak (hose or intake manifold), get a diagram for hose routing and check hoses - might be one pasted iside your hood, dealer may give you a free copy, or get a Haynes repair manual for your model/year #50010 84-99 jeep cherokee for about $15.00 at a locl auto parts store. If you don't already have one, I recommend highly - lots of useful info. Chilton also makes a manual, but it's not as user-friendly although it contains info that the haynes may not cover as well. Hope this helps.

Tiny
Grichards
Sep 8, 2007.
I bought a code scanner and even though it had scanned clean right after I had a new fuel pump installed it now showed codes for both O2 sensors. I knew that the front sensor was old so I replaced it.

Immediately the Jeep ran better but still surged under the same conditions described above. Sometimes it showed a O2 sensor heater code and sometimes it didn't.

To make a long story short. There is a fuse that is only used for the O2 sensors. This fuse was blown. I replaced the fuse and NO MORE SURGE.

Tiny
Yeloduster
Sep 17, 2007.
A FUUUUUSE? Just an inky-dinky FUSE? Man. I'll be filing this info away for future reference as to symptoms and causes - Thank you for responding! This is how we learn from each other! O2 sensors send a voltage to the computer based on oxygen content of exhaust gas, the computer adjusts the fuel/air mix. So, no O2 sensor input sends the computer into freaksville by sending zero volts instead of the 0 to 1 volts the computer usually sees.(Just thinking out loud). My manual recommends replacing O2 sensor every 82,500 miles.
A fuse? DUH. Makes me want to do a Carlos Mencia - " I'm a DEE-DEE-DEE!&Quot; Thank you for getting back!

Tiny
Grichards
Sep 17, 2007.
Maybe I didn't give enough information.

The fuse is for the O2 heater circuits. So the O2 heaters didn't work. Anytime the computer called for open loop operation and ignored O2 sensor input the Jeep ran fine. These times include cold start and WOT.

The problem presented itself when the computer called for closed loop and the O2 sensors were too cold. These times include idle and part throttle after warm up. If the engine load is great enough the exhaust stream temperature brings the O2 sensors to acceptable temperature and things work great.

In my case, I was in very rough country off road and a bush caught the leads for the rear O2 sensor and tore them out. While they were flopping around the heater wire shorted and blew the fuse. This started a long string of errors on my part trying to resolve this. I don't regret changing any of the components. The miles on the Jeep dictate that they be changed anyway. It was sure frustrating though!

Thanks for the ideas. They were good thoughts. Glad I didn't have to do that too!

Tiny
Yeloduster
Sep 17, 2007.
Thanks for getting back with a technician's - based response! I did some digging and found what causes closed loop and open loop modes, plus that the O2 sensor will operate in a programmed default mode/value if your fuse problem occurs. Not used to that level of detail - I'm an ex-submariner who used to work on communications encryption gear, and I appreciate hearing the details every once in a while! Lots of Luck!

Tiny
Grichards
Sep 18, 2007.