Bucking

[Danneman]

I have a situation that is hard to replicate for a mechanic without a long steep hill and smooth pavement, nearby their garage. I'm hopeful someone recognizes what I'm about to describe here…


1999 Yukon. Stock everything except fuel injectors (Delphi FJ10566) and exhaust (Flowmaster FLO-2010023 cats and FLO-817166 cat-back) and all in generally in very good condition, no other issues. Engine and trans have about 15K on them.


At around 38-45 MPH, in OD, when driving in leisurely fashion, vehicle "bucks" sporadically. Best word I can think of - a hard vibration throughout the vehicle. I notice it most when under strain (a hill, with A/C on, etc.). But this vibration is not at a constant rate, like X times/second consistently…at least not that I can tell. And it is not a high frequency vibration, like driving on a rumble strip. There is no rev change, doesn't seem like a TC lockup issue. Feels like I'm underpowered when it's happening, I can either wait until I get higher in the range and it eventually smooths out, or I can open throttle up hard and it drops gear, stops the bucking, and we get past it. I don't feel it driving in 3 setting, just OD.


Basically, if I'm at higher revs going through this speed range, I don't feel it. Casual driving, lower revs, demand on engine…that's when I feel it.


Maybe a miss, but I don't see how and…NO CODES.


Does this sound familiar to anyone?

 

[drakon543]

tps has a dead spot, maf needs to be cleaned, or you have an issue with the distributor. my tps on my 97 had a dead spot on its arc so there was always one spot in the throttle it didn't like. maf is just a random geuss but it can effect how well the engine runs. i say distributor as it almost sounds like it has a bad timing spot. maybe related to the cam gear cam sensor or maybe it didnt like the cap and rotor.

 

[iamdub]

My guess is something with the distributor, hopefully just cap and/or rotor and not the bushings in it.

There's a known problem where the CKP sensor contacts the reluctor on the crankshaft and causes intermittent misfiring under load. The fix is to shim out the CKP sensor and GM even has a kit for it. This may not be your problem but popping out the sensor is a really quick and easy check.

 

[Danneman]

tps has a dead spot, maf needs to be cleaned, or you have an issue with the distributor. my tps on my 97 had a dead spot on its arc so there was always one spot in the throttle it didn't like. maf is just a random geuss but it can effect how well the engine runs. i say distributor as it almost sounds like it has a bad timing spot. maybe related to the cam gear cam sensor or maybe it didnt like the cap and rotor.

New TPS after checking and not feeling confident with my multimeter that I was seeing even sweep, and new MAF after cleaning but feeling like perhaps it was damaged. So rule those out...

Distributor, hmm.

 

[Danneman]

My guess is something with the distributor, hopefully just cap and/or rotor and not the bushings in it.

There's a known problem where the CKP sensor contacts the reluctor on the crankshaft and causes intermittent misfiring under load. The fix is to shim out the CKP sensor and GM even has a kit for it. This may not be your problem but popping out the sensor is a really quick and easy check.

The misfires wouldn’t throw a code?

 

[iamdub]

The misfires wouldn’t throw a code?

Not necessarily. Not if the ECM doesn't see them as misfires and the "misfires" are just a result of the ECM's response to the wonky input from the CKP sensor. It's one bolt (8mm I think) to pop out the sensor and look at the tip for fine scratches.

 

[Danneman]

Not necessarily. Not if the ECM doesn't see them as misfires and the "misfires" are just a result of the ECM's response to the wonky input from the CKP sensor. It's one bolt (8mm I think) to pop out the sensor and look at the tip for fine scratches.

Sweet. I’ll check it out tomorrow, many thanks for the tip. Will post update when I have it.

 

[Danneman]

Well, here's a picture of the sensor. This would seem to fit the description of "fine scratches," I think? Do we think this could be my culprit?

 

[iamdub]

Well, here's a picture of the sensor. This would seem to fit the description of "fine scratches," I think? Do we think this could be my culprit?View attachment 218254

Those aren't the type of scratches contact with the reluctor wheel would cause. The sensor is basically thousandths of an inch from a toothed wheel that's spinning, so the scratches will be across the sensor tip in one direction and will easily show up. So, unfortunately, we're gonna have to dig elsewhere and deeper.

 

[Danneman]

Those aren't the type of scratches contact with the reluctor wheel would cause. The sensor is basically thousandths of an inch from a toothed wheel that's spinning, so the scratches will be across the sensor tip in one direction and will easily show up. So, unfortunately, we're gonna have to dig elsewhere and deeper.

Oh well, it was an easy thing to check out and now I have another thing that we ruled out.

So, if it were the distributor, wouldn’t I experience this issue at all speeds?

I keep going back to how I only feel this violent bucking in fourth OD, when relatively low in the RPM range. It’s like I am asking for power, and something is preventing it from giving it to me unless I open the throttle enough for it to decide that it needs to drop a gear. In lower gears, and at higher rpm’s in any gear, it seems to be ready for whatever I ask it to do.