The Stupid Engine Questions Thread

[Cale N]

Service Engine Light and Hard Shifting..

Specs
1994 K1500 Automatic 5.7

Symptom
Check Engine light on, hard shifting, engine stumbling at idle/stopped.

Relevant Info
The check engine light comes on only after cold starts. Before the light comes on the truck feels and drives perfectly fine, and can barely feel it shift, idles fine. About a mile down the road, the light pops on and all of a sudden every single gear shift feels like the transmission is about to go out. When I slow to a stop the engine stumbles (I think idle searching). If I stop the truck and turn it off for about 10 seconds and start it back up, the light is gone and it drives fine. It drives perfectly fine and the light doesn’t come back on, I believe this has something to do with the fact that the truck is already warm.
Changed plugs yesterday and didn’t have an effect, distributor is new and timing and idle are perfect.

Appreciate the help.

 

[PlayingWithTBI]

Check Engine light on

What code is it throwing? Even if the light is off it'll store the code for 30 - 40 key cycles, or when you disconnect the battery.

 

[thegawd]

Just a guess based on my experience. My burbs vacume booster went south. When this happens and you press the brakes, vacume leakes out of the plunger and into the cab. When it's really bad you can hear it and the longer you step on the brake, the more the vacume leaks, the vehicles ECM tries to compensate by changing the fuel to air mix and so when you let off the brake and stomp on the gas, the ratio is not right and it falls on its face.

Again this is speculation based on my experiences, I did not realize that my booster was leaking for a lot longer than when it was apparent. A proper operated brake booster is quiet and you really only hear the linkage move and the plunger push the piston, maybe some fluid movement but nothing else. It can be leaking enough all the time to mess up the vehicles ecm, then much worse when the brakes are applied.

A vacuum leak messes up air/fuel ratio and one way to test this is with a scanner while watching the short term or long term fuel trims. They will rise all the way up to %100 if you hold the brake long enough.

Of course if you can get the codes read it will point you in the right direction.


Take care.

Al

 

[thegawd]

I found the video that I watched that explained how to diagnose a failed vacume booster with the obd2 scanner. He goes on to R&R the booster but diagnoses it in the first 5 mins. It's the short term fuel trims and the oxygen sensors hes watching. In the video the hissing of the failed booster is not very loud. Mine was very loud when it finally failed but prior to failing I would get random O2 sensor codes. I bought a new set to install which I still will but I havent gotten a single O2 code since the booster was replaced. I guess the check valve on the booster or the rubber line could also be leaking...


Haha my burb has like 340000 miles on it now. LOL.

Al

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[SUBURBAN5]

That's the vid I watched when I replaced mine lol. It actually wasnt too bad either

 

[LTC_]

Where does this go? Found it laying on the manifold in front of the TB. It comes out of the same wire loom as the alternator wires.

You must be registered for see images attach

 

[SUBURBAN5]

Does it route to the compressor?

 

[PlayingWithTBI]

I can't tell from your picture but that should go to the temp sensor between cyl #s 1 & 3 for your temp gauge in the dash.

 

[LTC_]

Does it route to the compressor?

I don't see a spot for it on the compressor.

I can't tell from your picture but that should go to the temp sensor between cyl #s 1 & 3 for your temp gauge in the dash.

Ahh, that would make sense. The gauges (mostly not working) are a whole other story I'll get in to elsewhere, but the temp sensor you mentioned is hooked up to an aftermarket gauge in the cab and the cluster temp gauges is not working. I will have to get a correct replacement sensor and see if I can get the gauge in the cluster to register, but I'm afraid the cluster is having bigger issues.

 

[bigfutz]

Inside a 700R4, the 1-2 accumulator is a piston that, under increasing fluid pressure, compresses a spring inside a chamber. This controls the firmness of the 1-2 shift. Due to age, the piston often breaks off the center shaft and tilts off-center. The somewhat stuck piston causes the fluid pressure to build to a much higher level (and delay the shift) before it gives way and allows the shift to occur suddenly (to say, hard).

The transmission does not have to be removed to check this piston. With the transmission pan and filter removed, the round cover with three bolts attaching it to the case in one corner is the 1-2 accumulator. With the accumulator removed, check to see if the piston compresses freely into the chamber without tilting and binding. Any such restriction of free movement indicates replacement of the piston is required. They are not expensive, but a parts store may have to order it as opposed to having it in stock.

Be advised that there are one or two check balls that come out when the accumulator housing is removed. You will want to verify the location of these check balls before reassembly. Use transmission assembly gel to hold the check balls in place during assembly (they will not defy gravity and stay in place without the assembly gel).

If you're itching to see what's inside an automatic transmission, this is a first step in the journey, and again, this procedure does not require removing the transmission. Good luck !

Now do the 4L80e. Changed the fluid and filter on my 95 2500 and it helped the slipping and crazy shifting, but still shifts hard 1-2.