I have the MH listed in my signature i.e. Chevy 454 P series with I believe the 4L80E transmission. Recently experienced problem with not shifting into overdrive and intermittent Check Engine light. OBD scan shows bad EGR valve. Could this cause the shifting problem?

Not normally.What is the OBD code? 1994 did not have standard codes as the case in my 1993.

I guess it could if the EGR Valve is creating a vacuum leak. If it's throwing a code it's probably bad. Also, if you have more than 60K on it you will notice a difference if you replace the 02 sensor regarless if its throwing a code or not. They get lazy and affect performance and mileage.

enblethen wrote:Not normally.What is the OBD code? 1994 did not have standard codes as the case in my 1993.Don't know what the code number was. My mechanics code reader just said Bad EGR valve.

I'm not sure with the OBD1 computer but it may be possible that the computer sees not vacuum from the map sensor and won't allow it to shift into overdrive. Anyway I would try to fix the EGR first .

I've had EGR issues which turned into catalytic converter issues which then reduced engine power, resulting in inability to hold a higher gear...

Can bad EGR cause trnsmission problems? | Good Sam Community

MEXICOWANDERER
Explorer
Sep 18, 2017

It appears this transmission uses a vacuum modulator. A leak or a bad modulator itself will cause shift issues. Can shift issues interfere with the transmission going into overdrive?

Diagnostics may want to include taking vacuum readings at the EGR valve and at the modulator vacuum line connection. Correct vacuum signals eliminate the chance of a vacuum leak causing the problem but do not eliminate the possibility of having a bad modulator itself.

A pro-grade OBD scan might be wise. The above is just a hint about receiving confusing OBD scan results - where to look next.
allbrandauto
Explorer
Sep 18, 2017
yes you are right but vacuum leak is so minor it wont cause a driveabilty problem
allbrandauto
Explorer
Sep 18, 2017
ScottG wrote:
allbrandauto wrote:
A egr valve that has a vacuum leak will give you rough idle when you go down the road the egr valve opens up letting exhust into the intake to get rid of a gas called nox don't think this is your problem again cat. converter can cause this but you would also have no power is it not going into high gear or is the torque converter not locking up if its the torque converter not locking up could be brake light switch

It wont always give a rough idle.
Vacuum is only applied under certain throttle conditions (not at idle) and with the throttle closed, the engine will idle perfectly unless the EGR valve is stuck open - that is the only time it will cause a rough idle and that failure mode is rare. The usual failure mode of an EGR is that the vacuum diaghram gets ruptured. Then the vavle doesn't respond properly. Then at intermediate throttle positions, you will have a vacuum leak because the ECM is trying to apply vacuum to it and it's leaking.
ScottG
Nomad
Sep 18, 2017
allbrandauto wrote:
A egr valve that has a vacuum leak will give you rough idle when you go down the road the egr valve opens up letting exhust into the intake to get rid of a gas called nox don't think this is your problem again cat. converter can cause this but you would also have no power is it not going into high gear or is the torque converter not locking up if its the torque converter not locking up could be brake light switch

It wont always give a rough idle.
Vacuum is only applied under certain throttle conditions (not at idle) and with the throttle closed, the engine will idle perfectly unless the EGR valve is stuck open - that is the only time it will cause a rough idle and that failure mode is rare. The usual failure mode of an EGR is that the vacuum diaghram gets ruptured. Then the vavle doesn't respond properly. Then at intermediate throttle positions, you will have a vacuum leak because the ECM is trying to apply vacuum to it and it's leaking.
allbrandauto
Explorer
Sep 18, 2017
A egr valve that has a vacuum leak will give you rough idle when you go down the road the egr valve opens up letting exhust into the intake to get rid of a gas called nox don't think this is your problem again cat. converter can cause this but you would also have no power is it not going into high gear or is the torque converter not locking up if its the torque converter not locking up could be brake light switch
haydoracin
Explorer
Sep 18, 2017
Thanks for the responses. EGR will be replaced this afternoon. On another forum throttle position sensor was also mentioned as a possible cure.
time2roll
Nomad
Sep 18, 2017
I would fix the EGR before you take it to the transmission shop.
wnjj
Explorer II
Sep 18, 2017
With EGR problems you may have had your foot harder into the pedal, making up shifts unlikely.
TurnThePage
Explorer
Sep 18, 2017
I've had EGR issues which turned into catalytic converter issues which then reduced engine power, resulting in inability to hold a higher gear...
filrupmark
Explorer
Sep 18, 2017
I'm not sure with the OBD1 computer but it may be possible that the computer sees not vacuum from the map sensor and won't allow it to shift into overdrive. Anyway I would try to fix the EGR first .