TBI engine stumbles when cold

[mr heavy-chevy]

hello

1987 TBI with a manual transmission

******

low power. stumbles. falls on its face when cold.
i need to REALLY rev it and ride the clutch for it to break its inertia and build momentum.
just low power period.
but only when cold.
starts fine. idles fine. revs fine. runs fine.
just doest drive fine... when cold... it warms up and it's awesome again.

*******

recently i did the TPS. it was idling very high for a whole day. on day 2 i finally got the check engine light and replaced the sensor.
im amazed the ECM took a whole day to finally figure it out and throw a code.

i have no other check engine lights or codes at this time.

******


im thinking the EGR valve, most likely... maybe the MAP sensor...
apparently i cant count on the computer to tell me whats wrong.
any thoughts?

i may just start unplugging stuff to see if it gets better, worse, or stays the same.
otherwise im blindly replacing crap.



any help?

thanks

 

[Georgeb]

I would be looking at changing the fuel filter and then checking the fuel pressure.

 

[yevgenievich]

And engine coolant temp sensor. Check the resistance on it.

 

[mr heavy-chevy]

fuel is good. all new parts. and the injectors spray perfect.
if it was the coolant temp sensor then the truck would have trouble starting when hot. it assumes cold when it does not function.

i unplugged and blocked the EGR VACUUM line, and it seemed to fix the problem when cold.

after 40 miles of driving the check engine light came on.
i plugged the EGR vacuum back in and the light shut off in another 10 miles.
so i think its the EGR valve going out. or its somehow not tuned correctly?

 

[yevgenievich]

There is egr test procedure that you can go through. Also worth checking the coil, sometimes when it is failing, it has weak spark untill underhood temps warm up. Coil was a culprit on one of my old tbi vehicles having hard time starting/running when cold

 

[mr heavy-chevy]

there are 2 types of EGR vales for this motor and they're identical lol

so that could be it right there....

i have no casting or original part numbers to go on. so now i need to research OE application specs.... hooray (BTW theyre over $50 0.o WTF )

 

[mr heavy-chevy]

apparently one EGR is for manual transmissions and one is for automatic... so i think i found my intermittant hesitation problem.