Dreaded Random Misfire at high speed or under load- next steps?

[Tbruz]

Greetings,
My ride: 1997 Tahoe 2 door 5.7 engine with 211K miles.

My Issue: Getting a random misfire P0300 at higher speed( over 75 MPH) or when the engine is under load, uphill or passing with AC on. Getting and engine shudder as well flashing SES light which then goes solid.

Initially I thought the plugs were bad(and they were), so I replaced them with AcDelco Irridium 41-993
I changed out wires, distributor cap and rotor as well with no change; all AcDelco.

A dry compression check showed, 170 PSI in 5 cylinders, 2 at 165 and 1 at 160 so I can rule out any compression issues.

This weekend I replaced the 90K mile fuel spider and fuel filter with AcDelco parts, Idle seems a bit smoother but the high speed skip/misfire is still there.

I can go thousands of miles if I keep it under 65 with no incidents of misfire but as soon as I put my foot into it or go 75 or over I get the misfires.

took it to the local "professional" mechanic today and he "reset the timing"...basically he shut off the SES light because it came back on as soon as I got to the end of the ramp.


I'm really attached to this ride and I'd like to solve this issue.
I'm thinking of replacing the timing set/chain as I have a front seal leak and have lined up the parts to do that job so why not change timing set as I'm already in that deep.

I need ideas on what are the next steps I should take.

Thanks in advance
Tbruz

 

[OR VietVet]

What is the fuel pressure at idle? Attach a gauge and attach on a longer hose and tape to w/s and drive and see what the pressure is when under that load. How old are the crankshaft and camshaft position sensors. I keep hearing stories of those causing problems. Someone else will likely chime in with more recommendations. Almost forgot, did you check the wires at the pickup coil for any rubbed insulation? The timing moves in there and maybe causing a wire position to have a problem or contribute to a problem when under that load. Did you check for side play at the distributor shaft when doing the rotor?

 

[bsamole]

Greetings,
My ride: 1997 Tahoe 2 door 5.7 engine with 211K miles.

My Issue: Getting a random misfire P0300 at higher speed( over 75 MPH) or when the engine is under load, uphill or passing with AC on. Getting and engine shudder as well flashing SES light which then goes solid.

Initially I thought the plugs were bad(and they were), so I replaced them with AcDelco Irridium 41-993
I changed out wires, distributor cap and rotor as well with no change; all AcDelco.

A dry compression check showed, 170 PSI in 5 cylinders, 2 at 165 and 1 at 160 so I can rule out any compression issues.

This weekend I replaced the 90K mile fuel spider and fuel filter with AcDelco parts, Idle seems a bit smoother but the high speed skip/misfire is still there.

I can go thousands of miles if I keep it under 65 with no incidents of misfire but as soon as I put my foot into it or go 75 or over I get the misfires.

took it to the local "professional" mechanic today and he "reset the timing"...basically he shut off the SES light because it came back on as soon as I got to the end of the ramp.


I'm really attached to this ride and I'd like to solve this issue.
I'm thinking of replacing the timing set/chain as I have a front seal leak and have lined up the parts to do that job so why not change timing set as I'm already in that deep.

I need ideas on what are the next steps I should take.

Thanks in advance
Tbruz

 

[bsamole]

Both high speed and under load use more fuel. Have you replaced the fuel filter?

 

[OR VietVet]

He said he did the spider and fuel filter.

 

[Tbruz]

PNW, Thanks for the suggestions.
Fuel pressure at idle is rock steady at 50PSI; with my foot into, uphill and AC on it the pressure is 60PSI.
Immediately upon shutdown, the fuel pressure slowly rises(in about 60 seconds) up to 58 PSI
20 minutes post shutdown, fuel pressure is still holding at 58PSI.

EDIT: overnight fuel pressure drops to zero.
at Start-up , key on-engine off: pressure jumps instantly to 60. Key on-engine on, pressure drops to 50 PSI.
Monitoring pressure while driving, I can see a slight flutter +/- 2 PSI as the engine shudder/misfire kicks in.

Camshaft and crankshaft sensors are likely original, they have never been replaced in my 140K miles of ownership.
Was planning on replacing the crankshaft sensor as part of the timing cover/front seal Project anyways as its likely covered in oil form the seal leak.
I will look at distributor shaft play and pick up coil leads( those are in the distributor right?) tomorrow and get back at you.
Thanks again
Tbruz

 

[iamdub]

PNW, Thanks for the suggestions.
Fuel pressure at idle is rock steady at 50PSI; with my foot into, uphill and AC on it the pressure is 60PSI.
Immediately upon shutdown, the fuel pressure slowly rises(in about 60 seconds) up to 58 PSI
20 minutes post shutdown, fuel pressure is still holding at 58PSI.

Camshaft and crankshaft sensors are likely original, they have never been replaced in my 140K miles of ownership.
Was planning on replacing the crankshaft sensor as part of the timing cover/front seal Project anyways as its likely covered in oil form the seal leak.
I will look at distributor shaft play and pick up coil leads( those are in the distributor right?) tomorrow and get back at you.
Thanks again
Tbruz

At that mileage, slack in the dizzy gear is quite possible. Another strong possibility is the crankshaft position (CKP) sensor being too close or even contacting the reluctor on the crankshaft. This was a common issue with the composite timing covers introducing comparatively poor manufacturing tolerances where extreme accuracy was paramount. GM released a shim kit to space the sensor away from the crankshaft .010"-.020" (two .010" shims in the kit). It was hardly a problem on lower-mileage engines. But, worn high-mileage crankshaft bearings coupled with a flexing crank at high loads and RPM, the reluctor can get too close or even scrape the sensing tip of the CKP sensor. Take five minutes to grab a 10mm (or was it 8mm?) socket to remove the bolt, pop out the sensor and check the tip for tiny scratches in it. If they're there, add a shim and see if the problem is resolved. If it still happens but less frequently or at an even higher RPM or load, add the second shim and retest. When you're done, have a shop perform a crank relearn procedure. Note that even though the reluctor wheel may not actually be contacting the sensor, the extra movement in the crank may be enough to throw off the readings, resulting in misfires. I would say to replace the sensor if it has scratches, but I've never had a problem with NOT replacing it and just shimming it. Also, being where it's located, the wire harness is in danger of getting hot oil on it from the likely leaking seals. Something else to keep in mind.


Looks like an aftermarket shim kit here with three sizes: https://www.eficonnection.com/home/product/sensor-shim-kit

 

[Tbruz]

Iamdub
Thanks for the input; I'm already committed(part wise) to a timing cover replacement due to a front oil seal leak. I was planning on replacing the CKP Sensor as part of that job and will now add a Cloyes timing set too as a "while I'm already in that deep" action.
I guess I'm not objectionable to replacing the dizzy either as I plan on keeping this truck reliable for quite a while still.
Ill do the dizzy after I do the front seal/timing/CPK to keep the trouble shooting/resolution chain clean for future reference.
Tbruz

 

[iamdub]

Iamdub
Thanks for the input; I'm already committed(part wise) to a timing cover replacement due to a front oil seal leak. I was planning on replacing the CKP Sensor as part of that job and will now add a Cloyes timing set too as a "while I'm already in that deep" action.
I guess I'm not objectionable to replacing the dizzy either as I plan on keeping this truck reliable for quite a while still.
Ill do the dizzy after I do the front seal/timing/CPK to keep the trouble shooting/resolution chain clean for future reference.
Tbruz

Good train of thought. Don't forget about the CKP relearn after you have it back together. Even if it seems to run "fine", it likely won't be optimal. Ignition and fuel timing are kinda important. lol

There's an upgraded dizzy with a metal shaft available, if you weren't already aware. I don't mean a fully machined billet "racing" one. Just a stock replacement but with metal shaft.

 

[exp500]

I like to see fuel pressure 62-65. At 60 sometimes a problem. Lowest I've seen running well without any issues is 58.Same gauge different trucks. If you replace distributor, replace the cap with a Delco or United DCR820X only. Seem to be only ones that last. I recommend NGK plugs and Delco wires too. Once a misfire starts on these seems to come back quickly if only fixing one piece or generic parts.Have tried Packard wires with mixed results. 30000-40000 miles trouble free on a complete job. Use Dielecric grease on boots too. Look the radiator /water pump over close while out. Replace bypass hose on water pump. RTV corners and splices on timing set. Anti sieze on plugs.Inspect wireing on Crank sensor plug. Delco Crank sensor only. Dist requires scanner for setting position after install. No verification yet, But I suspect GM made some plugs in China for a short time, had a run lately of plug changes less than 8000 miles. Seems GM switched to NGK now. Switch to a 15-40 Quality oil- you have hi miles. Hope this helps.