Late last season and again last night my C5 (2000, 81K) periodically gives a really serious stumble on 2/3 acceleration and sometimes deceleration - it really bounces you in the seat like all cylinders miss a beat, not a weak sort of fuel related stumble.

It's just one strong "hit" and recovers instantly. Not every time either, it comes in bunches then disappears. The engine is at operating temp. Interestingly, the tach doesn't appear to see the stumble. No codes. Starts and runs fine otherwise.

Plugs and wires have under 20K miles, fuel filter is brand new. First tank of permuim fuel after winter strorage.

Oddly, and maybe related, the (second) new oil pressure switch acted up again. On starting it read zero, then gave a slow build to 60 pounds and sat there - no variation with RPM. On a restart it worked fine. It had been fine since I installed it and for 10 trips or so since.

I'm thinking a bad ground or maybe a cracked coil or more likely whatever controller/sending unit drives them all since the hit is so strong?

Is there a link to a map of the grounding points?

Ideas are always welcome!

Thanks!

 

I am having the same exact problem! I have 83k miles on mine. I went ahead and replaced the plugs and wires and that didn't help so I'm thinking I might go ahead and do the coils next but I hate to in case that's not the problem. Just like you it is only intermittent. Sometimes it runs like a beast and other times it falls on its face from 2nd to 3rd. If you find anything out let me know and I'll do the same for you.

 

When did this start happening? What mods have you performed before you experienced these symptoms?

 

My coupe was completely 100% stock with 70k miles on it when I first noticed it.

 

MIne is 100% stock as well. The only codes that came up afterward were related to the communications with the key fob - probably unrelated. I drove it last night and, of course, no problems at all.

Rohan, before you start throwing parts at it try some simpler and cheaper things first:

My car is old enough that the grounds might be getting corroded. I looked at this forum and found great examples of how to clean and freshen them. Just an afternoons work and eliminates lots of electrical problems for other people.

Also, Bill Curlee here has a great post on inspecting and repairing the electrical connections in the door - there is a serial communications wire there that frequently shorts out and causes no end of trouble. Great pictures. It's in the "sticky" section.

I'm also going to check the connections to the Crank Position Sensor. This is frequently the cause of large scale engine misfire/backfire/no-start issues. The wires are in a hot area - especially if you have aftermarket headers. A look there is worth it. Again, there is information and pictures on the forum.

Finally, it could ultimately be the Crank Position Sensor itself. $50.00 at the auto parts store. Pretty standard GM part, looks to be an easy replacement task.

I'd recommend all this before spending money on coils. Fresh plugs and wires is a good idea, of course.

I'll keep this updated. ANy other ideas are welcome!

 

Hey Scott,

Thanks for the tips! I just got done telling someone that they should try the easy stuff first and then look at me going and talking about buying coils. lol Anyway, I'll take a look at all the things you listed and see if that fixes it.

 

I had a similar problem twice in the Camaro.

First time was the Crank Position Sensor.
It would 'buck' like you were running over things sometimes.

Second time was a wire touching the header, and it burnt and shorted.

 

Hmmm...let me guess....you cut a hole to replace the oil pressure sensor.....

 

I did but slipped plate under it and didn't nick a wire - but I bet people do. What a mess that would be

 

That's good that there is no possibility of a nicked wire, because that would cause a similar issue....especially with the 5 volt reference.

 

Does this seem to occur at a specific/repeatable RPM?

 

Good question!

No. Not that I can really tell - it's not like a flat spot or anything I can cause to happen. Sometimes I'll be accelerating through 4-5K, at others just loping along at 2K in 6th and then "Bam" it bucks once like the earlier gentleman described.

Two observations that I think might be clues are that 1) sometimes I can feel it (although less pronounced) on deceleration and 2) the tach doesn't bump when it happens. I'm just not sure that the clues lead me to...

I guess I need more time with it.

 

Yes, I've taken some heat for using the cutting approach - it's potentially a disaster if you buzz through that 1 inch harness

Do you think the 5 volt reference might share a ground with the crank position sensor? (I haven't needed much in the way of shop manuals thus far - that's about to change!) I'm not that hot with ohm meters to begin with...

 

I have everything needed to troubleshoot any electrical issue. I also have a set of service manuals for a 99(same as 2000 electrically for the most part) that you can use if needed. I live in Hoffman Estates, so I would assume we are relatively close if you wanted to come by.

 

Wow, Thanks, I'm over in Arlington Heights. I should get my own service manuals - where did you get yours? I assume they are all over on eBay. Maybe you could show me the process behind testing...

I'll send a PM

 

Hey Scott,

On that last observation you made about it happening on decel with no jump on the tach. I have that problem too. Sounds like we are on the same path here. I'll start cleaning contacts and stuff this weekend and let you know if the problem goes away. I gotta do a WSW motor too this weekend. Got the intermittent stuff going on now. Fun Fun.

 

Well, it seems we're both chasing the same problem. I'll clean contacts, etc. this weekend as well and see what I find and let us know. I'm leaning toward the thinking the Crank Position Sensor is the issue - either the unit or the connections. I guess we'll see.

I have a 100 mile trip this afternoon to get home so that will give it plenty of time to act up!

I also have a Jaguar XK8 that gives me no end of electrical nonsense. This week it's taken to jumping into Limp Home Mode for a few seconds now and again.

 

Yeah, I've heard those Jags are about as bad as Corvettes when the electrical stuff starts. Thanks for the help man, We'll see what happens.


- Scott

 

I think most any late '90's to mid-'00 car has potential to be a electronic beast because of the age getting on them and the state of the art in electronics back then. I've had a lot of El Dorado's, and STS's of that vintage and they were always throwing codes.

My substantially pre-electronic truck? Mostly perfect at 180K miles.

 

Well, I put it up on ramps and took a look at the Crank Position Sensor but couldn't get a clear view - need to put it on stands I guess.

Nothing in the area looked out of place or over heated or anyting other than clean and stock.

I went through and cleaned and reset the ground connections on each front frame rail. Mine were a little corroded. I'll chase down the others one by one when I have time.

None of that really says anything, though. Other than I need a lift in the garage.

The car, however, is running flawlessly and has done so for 300 miles in all kinds of conditions.

The new oil pressure gauge gave me a slow-to-build reading for one ingition cycle, but that's it.