93 Grand Cherokee - Weird surge problem

Surging is usually a sign of running lean. I would check the operation of the O2 sensor(s) with a scan tool to make sure they are working properly. To do so you would need a scan tool that is capble of reading sensor values. You can also check O2 performance with a multimeter. It may be cheaper just to replace the O2 sensors from the start due to the high mileage on the vehicle.

I would check the fuel pressure as well just to rule that out as a possibility. The fuel pressure regulator increases fuel pressure under power conditions so it could still be low at cruise.

What engine do you have ? If you have a V8 they have a pan on the bottom of the intake that is prone to leaking, the only way to check it is to remove the intake.

Also check your EGR valve for proper operation. If it opens too far or at the wrong time that may cause a surge also.

Thanks for the suggestions. Is it possible the O2 sensor or EGR could be bad and not throw a code? So far I don't have any codes from the ECU.

It is a V8. I'll save pulling the intake as a last resort. Is there a method for a shadetree mechanic like me to test fuel injectors?

Thanks again!

sls

One more thing i didn't make clear in my original post. It's a cycling surge. If I hold the throttle steady as if I was trying to hit

1500rpm, the engine will cycle from 1300 to 1600 RPM. Each cycle (up- down-up) is about a second long. It does NOT do this at idle. RPM is a steady 700 or so. Pretty much anythin above 1000 RPM will do the 300 rpm cycle.

Interestingly enough, I replaced the throttle position sensor a few months ago. When I first started the vehicle, it idled at 1500 RPM. It did the same 300RPM cycle even though the throttle was closed. Of course after the computer "got used to" the new TPS it idled normally. It definitely appears to be RPM dependent.

Thanks! sls

Thanks for the suggestions. Is it possible the O2 sensor or EGR could be bad and not throw a code? So far I don't have any codes from the ECU.

Yes, especially a failing O2 sensor. The O2 sensor will be at the extreme end of it's range which is still within specs.

It is a V8. I'll save pulling the intake as a last resort. Is there a method for a shadetree mechanic like me to test fuel injectors?

If you wanted to remove the fuel rail and rig up a way to actuate the injectors you could check the spray pattern.

Thanks again!

sls

One more thing i didn't make clear in my original post. It's a cycling surge. If I hold the throttle steady as if I was trying to hit

1500rpm, the engine will cycle from 1300 to 1600 RPM. Each cycle (up- down-up) is about a second long. It does NOT do this at idle. RPM is a steady 700 or so. Pretty much anythin above 1000 RPM will do the 300 rpm cycle.

Interestingly enough, I replaced the throttle position sensor a few months ago. When I first started the vehicle, it idled at 1500 RPM. It did the same 300RPM cycle even though the throttle was closed. Of course after the computer "got used to" the new TPS it idled normally. It definitely appears to be RPM dependent.

Thanks! sls

Did you have this problem before you replaced the TPS ? Is the TPS adjustable, if so did you adjust it properly ? Sounds like it could be a TPS problem.

Usually the problem with the TPS is a bad connection, not a bad TPS. You only fixed half the problem with the new TPS. I would get some electronic spray contact cleaner and do out the plug and socket for the TPS first. The told us our TPS was dead in 1999, it was

The other sensor's connections can also benefit from this clean. I use dielectric grease on the plug's sealing skirt to help keep them clean.

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 'New' frame in the works for '08. Some Canadian Bush Trip and Build Photos:

Thanks for the feedback. The problem occured before I changed the TPS (one of the parts I threw at the problem) I actually did clean all the sensor connectors with radioshack contact cleaner (IAC, TPS, MAP) I also cleaned the washer and coolant reservoir sensors but they still show up bad at the most embarassing times on the VIC. :-)

How is the body ground strap? Sometimes when they let go, ground theft happens which can even be through the gas pedal cable, gear sift, etc. and strange things like intermittent sensors and intermittent computer signals happen.

Mike

On the 93 (4.0), it's actually a big honkin cable that's attached on the ignition coil.

For surge issues it can also be a plugged up fuel filter.

vbeergoggles.com

Good suggestion! I believe the ground strap on mine is on the passenger side inner fender near the battery. I took it off, cleaned it with a wire brush and reconnected. multimeter reads like .2 ohms from boltin fender to negative terminal of battery. Also, engine block to negative battery is about the same (my muntimeter always shows about .1 ohms from probe to probe) It still seems to surge.

It did appear to be corroded a bit so cleaning it was definitely a good move. The voltage meter on the dash seems to be a hair higher than it was.

Thanks again. Anyone have any idea how to test for bad injectors?

sls