I just blew off doing anything, and it magically went away.

But if I had to guess, my money would be on TPS. Why, you ask ? Well, backing off on the gas for a split second and reapplying immediately cures it every time. There's zero change in speed, so that action would not affect the VSS. It would however jiggle the TPS. Lazy man's logic.

 

As nuclearw said, put an ultragauge or live OBDII reader on it and look at the speed when this occurs. If it's not realistic or going haywire it is your VSS. Makes the most sense from a diagnostic stand point.

I saw a lot of people on here praising the ultragauge, didn't think I needed/wanted one. I finally broke down and I love it. Realized my truck was going 5 MPH slower than the dash gauge indicated. Can monitor my real coolant temps. I was running at 210 consistently then changed the fan clutch now I'm at 190, all the while the dash needle never changed position with a 20 degree difference. And I can check and clear CEL codes.

Bottom line, buy an ultragauge, you won't be disappointed.

 

Been there done that. Driving down the road, and the engine would cut out. Hooked it up to my UltraGuage, and every time it happened I got a warning telling me I was driving like 118 mph even if I was sitting still, so the VSS was kicking off the speed or rev limiter. Replaced the VSS & problem gone. When I took the old one out, it fell apart in my hands so I had to get a new replacement (look around as the prices I found were between $112 & $199 for the exact same piece).

 

looks like an ultra gage will be a good investment. So the ultra gauge speed mph reading comes directly from the the VSS?

 

That I'm not sure of, but would presume so as the vss passes that info on. I just know my ultraguage was giving a warning to the effect I was exceeding the maximum speed of 90 & said I was doing 118 mph sitting still. But yes, the ultraGauge is a must have!! They are $80.00 ish on their website.

 

Had same issue driving my 97 Disco 1 (180k, valve job at about 150k) about 3 weeks ago, long freeway trip, WA to Southern Calif. Felt like cylinder miss, intermittent. Had fueled in Oregon at cheapo station (regular). Got worse as got into 1/4 tank left. Thought might be bad gas / fouled injector so let tank run down, got Chevron supreme and put in a bottle of Techron concentrate, near Sacramento (no, I am not affiliated with Chevron). Ran fine the last 500+ miles of trip. Been running fine ever since (been using supreme), but will look into the VSS.

 

I had the same experience w/ my bad VSS... ran fine for a while then randomly freaked out (usually when it was a bit warmer outside for some reason.. )


I am not quite sure how the ECU calculates the speed, this puzzled me since it is getting data from the same sensor as the dash... Another way to test this is to simply disconnect your VSS.. It'll throw a VSS Code, and you wont have a speedo reading (use your GPS .. android phone has a "GPS TEST" app thats awesome, and gives you speed) but it will use other values to try and determine your vehicle speed for the engine purposes.. without the 90+mph reading to the ECU you won't have the engine loping

 

Missed this... been working a big DoD exercise... Busy busy.

I'm not sure. Any way of knowing without disassembly? My guess has always been valves; well, since replacing everything else. It's not a huge deal, but would be awesome to fix.

 

I had the same problem a couple of years ago on my '96 discovery. Intermittent power loss on the highway. Did a little research and found some who had steam cleaned their valves. Essentially the same as doing a seafoam treatment into the intake except using water. I did the treatment using water into my intake. I have never had the intermittent power loss issue since.

I am not a mechanic nor do I recommend any of the fixes I have used on my rover. This worked for for me but may not work for anyone else. Ever. Definitely try the seafoam treatment first.

If you do try the steam cleaning method you should do an oil change immediately after.

 

I thought valves too till i replaced the vss.... its dirt cheap to rebuild so a cheap test and if it hasn't it will go out soon..................


I read somewhere here that the 4.0's had the CC valves, my 97 seems to but i haven't done the heads yet so i could b wrong