96 GMC Sierra "skips" when shifting

[bcrooms]

My 96 GMC Sierra has been doing this for several months now, but its more noticeable.

 

When I am driving 60-70MPH, my truck (auto transmission) will "skip" when shifting to the next gear it seems. Maybe slip is a better term to use.

 

I'm not very educated on transmissions, but that's what comes to mind first.

 

I could be way off here, I'm just looking for thoughts on what the problem could be.

 

Any help is appreciated,

 

Thanks,

Brandon

[bcrooms]

Will do. thanks for your help on this. It's been an aggravation to say the least

[Jsdirt]

Yes, a misfire can occur at any time, under any conditions.

 

Ignition is one cause, compression & fuel (lack of) are other causes - clogged or partially clogged injectors, low fuel pressure due to worn fuel pump, stuck or leaking valves, worn rings, worn cam lobe, bent pushrod, broken valve spring, blown head gasket, and sometimes a plugged converter will do it too.

 

Vacuum leaks can also cause misfires. Hoses, EGR tube, & intake gaskets. On a V-type engine, you can have external or internal vacuum leaks in the intake manifold gasket. The latter is a tough one to find, but it can and does happen quite a bit. Easy to diagnose if you know what you're looking for - if you remove the oil fill cap, cover it with your hand, and it tries to suck your hand in, you've found that one.

 

Testing will narrow down which of the things in this list it is.

 

Water is a good way to check for both ignition problems, and vacuum leaks. Spray down the cap, rotor, and wires from end to end, snap the throttle, and see if you've got a misfire. Water will usually reveal any leaking spark. If you can get the nose of the truck into a dark area, it'll show up real good. Usually you can hear it before you can see it.

 

If there's a vacuum leak, water will get sucked in - usually you can hear that too. Helps to watch scan data while doing this. If you've got positive short-term fuel trims, and they start counting back down toward zero with the spray, you've probably found a leak.

 

If the plugs, cap, and rotor haven't been changed in at least 30k miles, and the wires have been on there for more than 60k, there's a good chance that could be your problem too.

[bcrooms]

The only light that is on is the ABS light and its been on for years.

 

I'm thinking it may even be a sensor or the regulator on the transmission. My father-in-law had an older Chevrolet once that had similar issues and it just need a new sensor

[Jsdirt]

Nice - keep us posted as to how it turns out.

[bcrooms]

I checked for vacuum leaks around the intake and the manifold last night, I did not have any success finding one.

 

I did go ahead and change out the fuel filter, but it didn't help as much as I thought it might.

 

Can I just get a code reader at AutoZone or does a mechanic need to use a real deal scanner on it?

 

The truck seems to get worse as it gets warmer.

[Jsdirt]

Shift solenoids fail on those all the time, but that will cause the trans to be stuck in one gear, and throw the light at the same time.

 

Wouldn't hurt to check for communication or body codes with a high-end scan tool - those codes won't set a light, and you'd never know they were there without the scanner.

[bcrooms]

Finally got around to check a few things on my truck. A few things that were found: low levels of rear end gear oil and transmission fluid was old

 

Rear end refill has helped quite a bit in the performance of the truck. I have not tested towing any trailers or wood splitter yet.

[bcrooms]

Yeah I was under the impression that my buddy was gonna do a diag scan with his high end tool but I don't think he had time to spend on it and not charge me.

 

He stays covered up with customers.

[bcrooms]

Plugs are recently new and the wires were just replaced at the same time the idler was.

[Jsdirt]

Is the tachometer rising before & during each shift, and then you feel the vehicle accelerate for a second as the tachometer drops, and the transmission fully engages the next gear?

 

Or, does it feel like the engine just cuts out in a jerking motion?

 

If it's the latter, it could be a misfire rather than a transmission problem.

[Jsdirt]

Any cheap scan tool can show live data - I'd highly recommend grabbing your own.

 

The ones the parts stores use typically only read & erase codes - not much else.

 

Getting worse when warm rules out the intake gaskets as a problem, and it sounds like you verified that already anyway. Usually things that get worse with heat are coils of wire, like an ignition coil. Hate to have you throw parts at this thing without knowing for 100% certain, but the only way you can be 100% certain is by hooking up a scope to the ignition primary or secondary. The waveforms will show if spark is dropping out randomly.

 

Most private garages (but not all) have an oscilloscope, or a scan tool with one built in, like the Snap-On Verus. Might be worth paying one for diagnostic time to hook it up and have a look. Would be money well spent at this stage.

[bcrooms]

After researching the symptoms I'm having with my truck and listening to friends who have had transmission problems, it sounds like my transmission may need repair or at least the torque converter.