OK here goes...I have a 99OBS Yukon and at speed it has a random jerk every so often. Did a fuel pressure leak down test (Good), new trans (so not that either), all new distributor, wires, spark plugs, ignition coil, cap, rotor. Maf cleaned, no vacuum or intake leaks, no engine lights or codes. Ran a very expensive Mac tools diagnostic scan tool live while driving with no misfires or drop in rpm (which is almost impossible). The truck literally jerks so hard you can hear it in the driveline and it sounds like the whole engine shuts off and starts back up in a split second. I can almost perfectly replicate the jerk if I speed up and slowly back off the throttle. Just not as noticeable of a jerk then. I have researched this until I'm ready to sell the truck... The only thing that came close was a guy on another forum said he had the same issue and after pulling the intake they found two of the spider injection lines backwards. Would the truck even drive if this was correct? I have ran bottle after bottle of fuel injection cleaner to no avail also in case I have the clogged poppet issue that GM did a recall on. And yes the motor has been replaced but no way to tell if it was just a short block swap or long block swap. No way to know if they had to change the intake to screw with the injectors. PLEASE HELP!
Random jerk in motor
- Thread starter sarge77
- Start date
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ISU-152
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I'd lean towards the tranny. Just cause it's new doesn't make it perfect
iamdub
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Pull the Crankshaft Position Sensor ( I believe it just needs a 10mm socket) and see if the tip has scratches in it from contact with the reluctor wheel. If so, and they're just barely there, get a shim kit for it. Last I knew, this was a dealer-only part, but it looks like it's available at least at Autozone (Duralast part number SU12931). Afterwards, you will need to have a CKP Relearn performed, which requires a TechII or other capable two-way scan tool. Or, being that the motor was replaced, maybe it just needs a CKP Relearn. Popping out the sensor only takes about 30 seconds. Oh- make sure the wiring, especially the connector, is clean and solid. That sensor's location puts the plug at risk of getting hot dirty oil in it if there's any leaks above it.
OK I'll take a look tomorrow.
Definitely not the trans... We replaced it because it started slipping and grinding. Truck was jerking before swap. Only thing that could make it jerk like that would be the tcc lockup. And it jerks in third also not just overdrive. So that rules out the tcc.
Nobody on the spider injection being swapped but still running? Guy that found his said it fixed it completely. Just don't want to pull intake if you couldn't tell they were wrong at idle or low speed driving. Seems like if you got one of those backwards and they inject fuel at a certain time it would skip all the time like a spark plug wire being wrong... Anybody?
iamdub
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This is my thoughts on it.
YukonandtheHOE
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Yank your tb off and take a look, if it has poppets then swap it for a updated one
That's what I was trying not to do... It's like 700 bucks for the updated injection. More wasted money if that's not the issue. I've already dropped way too much in this already trying to make it a daily driver that was dependable. I'll lose money like crazy now if I sold it... Plus the injection
iamdub
Full Access Member
The 4.3 in my S10 would do the same thing. It would feel like the engine died for a split second and not just a single cylinder misfire. It did this after I had floored it (accelerating, merging, passing...) then went back to a steady cruise. It would do it just once. If I gave it a little throttle, like just enough to accelerate a bit but not downshift, then settled at a steady cruise, it would do it once more. I could stop, shut it off and restart it and it would fix it until the next time I floored it then cruised.
Your 350 is essentially the same in the front and probably has the same timing cover with the sloppy manufacturing tolerances. The CKP sensor doesn't have a very wide tolerance for spacing from the crankshaft. Some covers put the sensor too close, others too far. Most were too close which is why the spacer was introduced. Of course, there's also variances in the CKP sensor manufacturing itself to further exacerbate the problem.
With my issue, all was fine until the crank moved enough radially at high RPM to contact the sensor. My guess is that this threw it out of whack and the PCM kinda resorted to a base map to keep it running normally. Applying throttle causes a change in RPM and the PCM alters the timing, so this is probably where the hiccup occurred. It's like the PCM just cuts all ignition activity then resorts to it's base map which gets it running again. This happens so fast because of the processor speed of the PCM, so it doesn't seem like a single- or multiple-cylinder misfire and more like a quick "key off/key on" action.
If you still have access to that MAC scan tool, have a CKP Relearn performed after you inspect (and space out if necessary) the CKP sensor. You may be able to fix this with no parts cost.
Your 350 is essentially the same in the front and probably has the same timing cover with the sloppy manufacturing tolerances. The CKP sensor doesn't have a very wide tolerance for spacing from the crankshaft. Some covers put the sensor too close, others too far. Most were too close which is why the spacer was introduced. Of course, there's also variances in the CKP sensor manufacturing itself to further exacerbate the problem.
With my issue, all was fine until the crank moved enough radially at high RPM to contact the sensor. My guess is that this threw it out of whack and the PCM kinda resorted to a base map to keep it running normally. Applying throttle causes a change in RPM and the PCM alters the timing, so this is probably where the hiccup occurred. It's like the PCM just cuts all ignition activity then resorts to it's base map which gets it running again. This happens so fast because of the processor speed of the PCM, so it doesn't seem like a single- or multiple-cylinder misfire and more like a quick "key off/key on" action.
If you still have access to that MAC scan tool, have a CKP Relearn performed after you inspect (and space out if necessary) the CKP sensor. You may be able to fix this with no parts cost.
