GM 4l80e trans vibration in O/D when towing

[rognelson777]

Towing with a 1998 motorhome, has 7.4l chevy towing a 28ft champion trailer. When cruising at 65 mph on flat ground all is well, as soon as we hit grade, vibration starts, feels like the converter cannot decide to unlock or trans wants to downshift. If I depress gas pedal to full throttle it will downshift and vibration stops, then it seams to upshift but not lock up converter, when back to level ground converter locks up and all is well. no other trans issues. Motorhome has 58,000 miles.

Been researching options, 1 is to upgrade converter to TCI/hughes/B&M
2 is to rebuild trans.
3 Fast has a tuner / transmission re calibration programmer that will change transmission settings.

also thinking off upgrading exhaust for a little more horsepower and torque.

If all of the above is necessary, then may be time to upgrade motorhome to newer model or diesel.
Any recommendations ?

[Dave Noll]

I saw a local shop sell a customer a trans rebuild for the same issue your having. When the customer got it back it, did the same thing. In his case the engine was missing under load. Common issue's would be carbon tracked plug, plug boot, weak injector if it has 8, coil output if it has 8, or plug wire etc.

[Alan Roehrich]

Make sure it does not have an engine miss.

In the event that it does not have an engine miss, have it rebuilt by a shop that has someone who has full knowledge of all of the fixes that Sonnax has for that transmission, and will install them correctly. Have the converter rebuilt by ACI in Old Hickory TN, just outside of Nashville.

[Rob Petrie E395]

Put a fuel pressure gauge on it and make sure it holds pressure after its shut off. If it does not pull the upper intake so you can get to the vacuum line going to the fuel pressure regulator and the injectors. Disconnect the vacuum line from the fuel presure regulator and cycle the key so the pump runs and builds pressure in the sytem. If fuel comes out the vacuum port the regulator is bad. If no fuel comes out take the injectors loose and see which one leaks fuel with pressure on the system. My bet is Cyl #4 injector is leaking maybe others too. No idea why but of the several I have done #4 leaks everytime. The regulator and injectors are about 80 bucks a piece at napa. If the regulator is leaking after you replace it recheck and make sure it cured it and there is not one or more injectors leaking before you reinstall the upper intake. Also replace the small vacuum hose with a new piece of hose if the reglator was bad.

If all is well here and its holding pressure take a good look at your engine mounts. If it has a bad one and they are like the pickups let me know. And I will pray for you. They are a SOB to change.

Hope this helps

[rognelson777]

Going to put new plugs wires and cap on. Injectors were checked last year by a fellow drag racer and they were okay. will check regulator.

Thanks for info. I will follow up when work is complete.

[Dave Noll]

Just curious if you got your Motorhome fixed yet ?

[rognelson777]

Update, New plugs, wires,distributor cap, rotor and problem is gone. I OHM'd all the old wires and plugs and all matched the new wires and plugs.

I guess something was breaking down under full load.

Used over the weekend towing in virginia and all is well. Other crew members could not believe the difference.

side note, when checking fuel pressure at idle, it may be low, so give it a little rpm and it should come up to spec range of 54 to 57psi. Mine was 47 at idle but as soon as I touched gas pedal went to 54psi and stayed there, even under load.

thanks, class racer

[Ed Wright]

Next time somebody has an issue like this if you have somebody with a Tech2 scan tool ride with you and watch the mis-fire monitor. If missing it will show it and even which cylinder(s).

[Keith 944]

good ideas, my 99 454 m/h does the same thing intermittent miss. changed plugs, wires, cap etc. ran great for year, think it is due again...

[Ed Wright]

Originally Posted by Keith 944

good ideas, my 99 454 m/h does the same thing intermittent miss. changed plugs, wires, cap etc. ran great for year, think it is due again...

I have found a lot of that is dirty injectors. And no, pouring something in the tank won't fix it. LOL