th400 causing idle surge in gear?

[Michael Sammy]

Hi,

I had posted a while back about my 400 sbc having a surging idle in gear, but in park it was perfect idle. So I did what some members said and disconnected the th400 vacuum line and put it in gear and drove it around the block and the idle was perfect in gear.

What does this mean? Is it the vacuum modulator on the transmission that is bad?

Thanks

 

[CorkyE]

Is it the vacuum modulator on the transmission that is bad?

Thanks

It could be, did you cap the vacuum port when you took the line off? You could also have a break in the vacuum line itself. Usually when the diaphragm on the modulator goes out, it will suck transmission fluid into the engine and you'll have a good amount of white smoke. If you can borrow a MitiVac, that will really help trouble shoot this problem.

 

[Michael Sammy]

It could be, did you cap the vacuum port when you took the line off? You could also have a break in the vacuum line itself. Usually when the diaphragm on the modulator goes out, it will suck transmission fluid into the engine and you'll have a good amount of white smoke. If you can borrow a MitiVac, that will really help trouble shoot this problem.

I cap all open lines whenever I am messing around, so it was not due to open vacuum line. I did MityVac the trans line today and it help pressure in gear and in park, both tested with engine running and engine off. There is a little bit of white smoke from the exhaust though which I was looking at past recording of my vehicle and there was no smoke before.