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

[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.

[bcrooms]

Tachometer stays in the same position, speed stays the same. I'm not 100% sure it is during a gear change, it just feels that way.

 

It feels more like the tires lose grip on the road or I hit a bump in the road, but the road I drive on daily has been freshly paved and is 4 lane.

[bcrooms]

Seems to be misfiring, I had the idler replaced as it was locking up and needed replacing.

 

I have read wheremisfires can occur at any speed, in any gear, etc.

 

Could it be the ignition coil or distributor or something else?

[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]

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

[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]

Any more ideas? truck hasn't improved much. I thought the fluid changes might help but it hasnt.

[Jsdirt]

Have you thrown a scan tool on there and checked the fuel trims, and other data? Is the "check engine" light on? Did spark hold up after the water test? Any vacuum leaks?

 

Can't help you from here without a little more info.

[bcrooms]

Have you thrown a scan tool on there and checked the fuel trims, and other data? Is the "check engine" light on? Did spark hold up after the water test? Any vacuum leaks?

 

Can't help you from here without a little more info.

 

I haven't tried a scanner yet. My local autozone has code readers.

 

The only light I have on is the ABS light and its been on since i bought the truck. Previous owner said it was always on as well.

 

How would I do a water test or check for vacuum leaks?

 

The best way I can describe it now is that I'll be cruising at 55-60 MPH and the truck will go to overdrive I assume, and the engine RPMs will decrease as a result of the gear change, its AFTER the engine RPM decrease that the truck bucks, jerks, whatever you wanna call it.

 

And under any weight on the a trailer hitch, it does the same constantly no matter what speed. Makes me afraid to haul anything that requires a hitch.

 

It has new universal joints, fuel pump, rear end gear oil, transmission fluid, plug wires, distributor cap, idle pulley, steering coupler

 

I have not replaced the front end gear oil or the transfer case fluid yet.

 

I'm not an expert by any means, but to me it points to something in the drivetrain area based on the rough ride, bucking at odd speeds and such.

 

I know it can be anything, but it feels like its towards the rear of the truck.

[Jsdirt]

Ahh, ok - that info sheds alot more light on what you've got going on here.

 

I see you replaced the cap and wires, but no mention of the plugs. Your symptom is classic of worn spark plugs. They will fire fine in open air, or under light loads ... but as soon as you put a load on the vehicle, the ignition system does not have any reserve power to clear the large gap (caused by normal plug wear). There's a formula for how many thousands of volts it takes for spark to jump .030" at xxx PSI ... and when pressure (or the gap) goes up, the voltage required goes up BIG.

 

Before any testing, remove and inspect the spark plugs. See if that center electrode looks rounded or short. If it does in any way, throw it away and buy a new set. If your curious (as I am) as to how much of a gap you have, auto parts stores usually have cheap plug gap measuring tools there.

 

I had a vehicle with the exact same symptoms once. Guy refused to believe the issue was as simple as plugs. I told him it doesn't hurt to check, and with that, yanked the first plug on the front bank (Mercury sedan). The gap was so big, I could only measure with an allen key! That came out to .100" or ONE TENTH of one inch! ENORMOUS!! They should be no more than .045". Some vehicles ran them at .060", but that to me is too much - they run fine at a smaller gap, and that gives the system plenty of reserve for any future problems.

[bcrooms]

I had the plugs replaced after I purchased the truck from the previous owner. That was about 4 years ago. I can't recall the truck having any of the symptoms back then as it does now.

 

I will inspect the plugs tonight to verify the gap.

 

I looked up the plugs on AC Delco's site and all of their plugs for my 96 K1500 are pre-gapped to .060" from factory. If I read right, they recommend not adjusting iridium plugs.

 

Do I need to use platinum or double platinum plugs?

[Jsdirt]

Platinum lasts longer than a regular plug. "Double" platinum is a marketing gimmick, IMO.

 

I run iridium plugs in my Silverado - same as OE. All depends on how long you plan on keeping the truck. Iridiums can last up to 100k miles, but of course, they cost ALOT more money.

 

Don't try to adjust the gap on iridium plugs, or any plug with a fine wire (thin) center electrode. If the center electrode is wide (like they've been for the past century) you can adjust it all you want.

 

If the plugs from 4 years ago were economy plugs, 4 years is about double their lifespan if you put more than 12k miles a year on the truck.

 

Plugs are cheap enough, so doesn't hurt to toss a set in and see if it helps. If that doesn't work, then we can move to testing. Just a simple spray bottle of water, or a soda bottle with a hole in the cap works great for water testing. Basically your just dumping water on components until the engine stumbles, runs rough, or dies to pinpoint a problem. Air leaks will make all kinds of noise when water contacts the leak. Safer than dumping flammable liquids all over the engine, and water is cheap.

[bcrooms]

True,

 

I honestly don't remember what type of plugs were put in it. It's probably a good bet to change them again anyway.

 

I'll go with iridium.

 

I don't drive my truck every day but I have put roughly 10K miles on it since I've owned it.

 

It's mainly for hunting,hauling, and the occasional cruise around.

 

It has started to idle rough while in drive and reverse but not under any acceleration.

[Jsdirt]

Those symptoms point towards a vacuum leak. Intake gasket leaks were common on those. Can pour some water along the gasket making surface of the lower intake manifold and see if you can hear it getting sucked in, or hear the engine start to stumble, or change RPM.

 

Sounds like the problem is flip flopping though, which is more than likely an intermittent electrical problem. Tough to diagnose anything over the net, but intermittent are tough in person ... let alone over the WWW.

 

If the truck runs real bad when first started in the morning, but only at idle speeds in gear or in park, then seems to get better as the engine warms up, that's a classic intake manifold gasket leak. Fuel trims and o2 voltage on a scan tool will point that out. If the trims are going positive (meaning the computer is adding fuel to compensate for a lean condition), that's a good indicator as well, especially if the long term trim is much over 10. You want to be watching your short term trim as soon as the truck goes into closed loop - if that is also way positive (over 10), you've probably got a big vacuum leak somewhere.

 

Pretty much need a scan tool these days to do any kind of troubleshooting. Otherwise you have to throw parts at it, which could've bought more than one scan tool in some cases.

 

I miss the old days! Points, and a carburetor - things were easy then!