RPMs quickly changing up & down from 2000 to 1500 with a slight jerking motion when driving at 40-50 MPH. After getting passed 50 MPH it returns back to normal. I recently purchased this 1998 Disco 1 about a month ago. Does anyone know what this might be? New to forums. Thanks in advance

 

Have you given it a full tune up & cleaning? IAC needs to be clean, TB, and fuel filter. Whenever I had an RPM or speed surge it was either the CPS or a bad fuel pump. My XD would surge occasionally when on the highway, and it would usually go away, but once I had to pull over and let it sit. Once it cooled it ran fine and I immediately knew it was my CPS. Grabbed one on Rock Auto for cheap and it's never surged again.

 

I’ve done a tune up but in the process I also noticed leaks so I changed out all the high pressure hoses so I didn’t get a chance to finish a complete tune up. Also noticed that the surge from 2000 to 1500 happens about 99% of the time while driving at 40-50 mph, doesn’t matter if it’s hot, cold, up hill, or flat even road. I will clean out all the parts you mentioned & see how that goes. Thanks again.

 

Change the CPS Rock Auto has em dirt cheap. Takes about 30min to do and that will probably take care of your issue. On a GEMS D1 the wiring insulation can crumble or the sensor itself just starts going out and cause odd running issues. I think the last CPS I got from Rock Auto was like 7.00 vs the usual 30.00.

 

I guess I will give a new CPS a try too. That way we'll have two experiments to compare.

For my 96, it has had those same symptoms for the 8 years I have owned it (7 years of driving, mostly it is the wife's winter vehicle). It is caused by the automatic transmission shifting up & down right at that spot, in and out of lock up. I have learned to avoid the "bucking" by feathering to the gas pedal or hitting the gas. It usually hits me while accelerating to merge onto the freeway. It happens more going up a slight incline. Once speed is over about 55 mph, there is no more problem, and no problem after it down shifts going less than 45 mph. And no problem during very light throttle acceleration or heavy throttle.

My CPS is new from (A-B maybe? Don't know the brand) when I rebuilt the motor after I bought the truck for $800 on my birthday (best present I ever bought myself). The PO had intentionally shorted the CPS leads together, and the old sensor pin was slightly bent. I straightened the reluctor and we've put around 22 kmiles on it since then.

There are no other symptoms. Starts right up every time. Gets around 14 - 16 mpg. No codes, except SRS is always lit or flashing. O2 sensors start to modulate about 60 seconds after starting. I reset the adaptive values a few weeks ago, and that actually seemed to help, but it still happens. Probably the best improvement was from adjusting the kick down cable very carefully, according to specifications, years ago.

Anyway, I am all in on a $7 experiment, even $30 would be a bargain, considering the $3000+ in parts I've put into it, and considering a tankful costs over $70. Wish me luck.

 

Mine was fine at 65-75MPH cruising down the road. It never tossed a fault, but every single time I got into traffic and had to slow down after being at a constant 65-70MPH it would surge/bog, then if I gave it throttle it would sometimes stumble and clear up and other times the RPM's would drop. Meanwhile the transmission wasn't acting right at all. Upshifting or downshifting very nasty like the kick down was totally out of adjustment. It always happened whenever I was passing thru Houston, TX and had to slow down for traffic. First time I felt it and thought I got hit by a heavy crosswind. Then it did it again and when I touched the throttle it did not respond smoothly. Then it cleared up as I got thru Houston, TX that time with little traffic. I honestly had just filled up like 30 miles before and I thought I'd bought some bad gas. Second time it was hot out for starters, then the traffic was very bad. It even sputtered once and I figured it was going to die, but if you let off the throttle and coasted for a bit it would usually clear up. This time it did it all the way across Houston. I pulled over (of coarse I was on a deadline to be somewhere.....) checked every single thing and zero issues. I had it turned off maybe 15min while I checked stuff. Got back out on the road, and it never did it again for the entire trip. When I got home I ordered a CPS as I had already changed the fuel pump with zero change, the IAC, the ECU, and even the TPS (all spare parts I have in my trail box). Next trip up to Austin, TX I tossed the new CPS in my trail box and hit the road. Got to Houston, and zero traffic so I never really slowed down from 65MPH. However once near Brenham, TX there was an accident and I had to slow down and sure enough it went back to it's surging BS. I got to my friends house (LR Master Tech) and he asked how the XD did on the drive up. I explained the surge and he asked any codes? I said no and he instantly said change the CPS. Next morning I changed it and since then I've made probably 5 trips up to Austin, Marble Falls, Taylor, TX with zero surges/bucking or weird shifting.

My other buddy had a superb 99 D1 SD and it would run great until it got warm. Then it wouldn't rev past 3k without wanting to die. Keep it under 3k and it was happy. He finally swapped out the CPS and bingo the issue went away. He also didn't have the cover protecting the CPS from the elements so I could see how heat would bother it.

 

Just got back from a short test drive with the new CKP sensor installed, and happy to report no unstable shifting, so far. I did notice, sometimes it shifts at 45 mph and sometimes at 50.

I removed a thin gasket from the old set up. I think is was a TPS sensor gasket that I installed years ago because it came with the gasket set and I didn't know what else to do with it. My searches showed no gasket belongs there. So the spacer for my new sensor is a bit closer and grounded better. I also tightened the bolts tighter than before. If it starts bucking again, I'll report back. Otherwise, I'm good to go!

 

Drove to work today, about 50 miles round trip, city and highway driving, no unstable shifting even though I tried my best to make it happen.

 

Glad to hear it worked for you JohnZo. That gives me hope. Unfortunately I haven’t had time to change it out, the winter here in Spokane hit hard & we had a leaking rear sunroof I had to change out. Plan to get it changed out once the weather gets a little better. I post once I get it done.

 

Well, here's an unfortunate update. Bucking still occurring. While accelerating a bit more than a light to medium throttle, same symptoms. Can't make up its mind right around 45 mph, shifts up and down. Seems to loose power when it finally locks up, even without the bucking.

My next step is to examine the reluctor ring more closely for bent tabs, but it starts and idles fine, so not a big priority for me. Anyway, now I have a known good spare CKP sensor.