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I have a 1978 Firebird with a 350 Chevy engine and a TH400 trans. The engine and trans are not original and I beleive to be pre-1975.


The problem I am having is that, when the car has cooled off, I can start it and it will move about 1 block, then the engine races and the car stops. The transmission fluid is clean and does not have any burnt odor.  If I let the car cool off, I can start it and move another block, then it repeats its none movement problem. I have been told that the front pump in the transmission is bad; I have worked on cars for 45 years, but have no experience with transmissions. I have looked at diagrams of this tranny and believe I could probably do this service but would like more opinions as to what the problem may be.

All responses are helpful and appreciated.

                                                        Steve Wilson
                                                        Lansing, MI  

first verify the fluid level in the transmission with the engine idling in park...

the fluid level should be at the proper full mark... to verify.

stick a long piece of NON FLUX COATED.. welding rod into the dip stick tube.. all the way to the bottom of the pan... the fluid level idling in park should be about 3 inches to 3.5 inches up the wire.. depending on the pan depth.. usually just above the top of the pan gasket...

could it be a deep pan.. with a shallow pan filter????
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how is the vacuum modulator tube and hose sections...

pull it off at the back of the manifold..   use a hand vacuum pump to create suction in the tube going down to the modulator... it HAS TO HOLD VACUUM.. No leaks...

i normally pull the vacuum hose off at the transmission.. verify that i have a MASSIVE vacuum leak when i do that.. lots of vacuum flow..   i had one car that was a swap.. somebody put a vacuum TEE into the hose to the modulator..  right over the heat riser crossover in the intake.. the bottom of the plastic vacuum tee melted and was pulled in blocking the vacuum flow.. so there was limited vacuum to the modulator..

when the modulator is not supplied with proper vacuum signal .. it will act really strange...  kinda like being in neutral...

i wonder if somebody has hooked up the vacuum modulator hose thru a ported vacuum switch.. so the vacuum is shut off when the temp comes up...  i have seen that...

the modulator tube needs to be 1/4" steel tube with only a short rubber section at the top and bottom to stop the rubber hose from absorbing the changes in vacuum and delaying the signal to the modulator..

i have found the vacuum connection on the back of the intake clogged with carbon...

i have also found vacuum modulators hooked to the EGR port on the carb.. /Ported vacuum.. no vacuum at idle..  vacuum at increased engine speed...  when it comes off fast idle.. it will loose vacuum to the modulator..

you might also want to check that the transmission actually changes gears with the rear wheels in the air at the shop..  that the governor is not failing..


the transmission professionals will have a LOT more info.. so run these tests and post what you find..


i am sorry guys for stepping into a transmission post.. but i though i could add some info...  

waynep71222,

 Thanks for the info. I,ll check those items in the next few days and repost.


                                                           Steve Wilson  

It sounds like a case of the filter plugging up with debris- either metal or clutch material.  When you let it sit,the debris siphons back out of filter enough for the pump to pull fluid through it until you run it again.  If you pull the pan you'll probably see the evidence.  I'm in GR if that helps.  

I agree with boone
If the filter is cologed after a short run you should here a high pitch whining noise that sounds something lika power steering pump whine
If you have a loud exaust system you may not here it  

Boone1 and Bbuss

i think you have it...  makes perfect sense to me..  

TH400 filters sometimes just clog up with age even if there is no debris in them. I have seen them clog up just like this one and after changing the filter they run for years with no trouble.  

I think the filter media collapses in on it self
Probably cheap forign filters  

Thanks to all of you for yoyr responses.  I checked the vacuum(sp?) and it is good and strong and the modulator holds vacuum. Tomorrow I'll drop the pan and change the filter; will post results.

                                 Steve Wilson  

If the filter is cologed after a short run you should here a high pitch whining noise that sounds something lika power steering pump whine.  
I heart which I could not help him

THANK YOU THANK YOU THANK YOU ALL!!!!

I changed the filter and fluid as recommended and the problem seems to be solved.

Again, many thanks!

                        Steve Wilson
                        Lansing, MI