I told you the number backwards the block is a 350. I met up with the guy I
bought the truck from and he told me the block is a 350 out of a 79 van.
The heads intake and throttle body are off the old 305 that was orignally
in the 88 truck. The computer is from the 305 as well. The tranny is not
the orignal either it's a turbo 400 out of god knows what and has no kick
down. I'm really confused now. Do you think this can work. Or is there
something they might have done wrong? Or doesn't match up.
Awww hell you're in bad shape man. The 305 heads will NOT work with the 350
block. Combustion chamber is too small as are the valves. What's happening
is the increased compression due to the teeny combustion chambers is causing
spark knock, which in turn is causing the ECM to retard the spark, giving
you the hesitation and shitty performance. Add the fact that she's not
getting enough gas due to the smaller injectors and it's going to run lean
and make the spark knock even worse.
The chip inside the ECM (PROM) needs to be reflashed for the bigger motor,
different tranny, etc. and you need bigger injectors.
Easy fix: Find a decent set of 350 heads, an intake for a 4-barrel carb and
a 4 barrel carb and run her carbed. You'll never get that thing to run
right with EFI.
The TH400 transmission doesn't have a cable kickdown like the TH350 or the
TH700R4, just a lone vacuum line running from the intake manifold to the
vacuum modulator on the tranny.
Sorry to be the bearer of bad news, but that thing is a giagantic cluster
Well it can't be a turbo 400 than because there is a spot to hook a wire
up. Its a flat connection and I believe its a green wire with a female
connection that slips right on. I did this and it kicked down but it when
it jumped into 3rd it would jump back down to 2nd when I let off the gas.
Also if the engine was made to have a higher compression should it still
read about 120 for a compression test. I'm also going to check the fuel
pressure what should it be running at?
Let's take a simple approach at this:
1. Count the number of transmission pan bolts and post the number here.
Ten bucks says it IS a TH400.
2. 120 psi of compression is shit on that motor and indicates it's time for
a rebuild. 150 psi is healthy.
3. Fuel pressure should be 9-13psi.
4. Too late to get your money back?
There are 13 bolts. I don't know what the compression is yet I was just
wondering what it should be at. Tomorrow a friend is stopping by with a
computer to pull the codes off the engine then we are going to check
everything possible. I'll let you know what we find. Is the throttle body
for a 350 different from a 305? If they are can i just change the
injectors? The body and interior are mint so even if I have to drop a new
engine in it's still worth it. Be sides I'm a diehard chevy fan that
doesn't give up easily. Thanks for all your help so far!
That's a TH400.
Compression should be at or above 150 psi. You might be pushing 170-180
with those heads though.
You can check your codes with a paper clip:
TB is the same on 305 and 350, but the 350 injectors are bigger.
So can I change them. Or do I have to get the TB replaced. Its probably
easier just to change the whole TB. I have tried the wire trick but my
friend has the computer which is easier hopefully. What is the wire
connection for on the tranny, its on the drivers side and when hooked up
make the tranny kick down when I step on the gas.
Check the vacuum line from the back of the intake manifold to the vacuum
regulator on the passenger side of the transmission case. If the vacuum
line is old/cracked/leaking it'll mess with your shift points.
Looked into truck specs it had a tranny with overdrive that wire is more
than likely hooked in to the computer and the computer thinks 3rd gear is
overdrive and shifts down to 2nd thinking its 3rd. I need to find something
that had a 350 with TB and 400 turbo tranny that is run by computer so it
can send the right signals. Any ideas?
The computer doesn't have anything to do with the transmissions on these
older trucks. The ECM didn't start becoming involved in transmissions until
1993 in full-size GM trucks.
The cable that <was> on the truck when it had the 700R4 was just a simple
cable that ran from the throttle linkage to the tranny; no sensors, no
computer "knowing" what gear it was in, etc. The computer could guess what
gear it was in based on VSS, TPS and MAP sensor signals and distributor
feedback, but it never "knew" 100% what gear it was in.
Did you check the vacuum line? How's the fluid level and quality in the AT?
The vacuum line is fine. I was told I would have to wire up a switch under
the gas pedal in order to get the tranny to kickdown when needed. Any ideas
as to what I wire this switch into? I also checked the compression its at
160 the timing is also dead on. GM told me to go with a 350 TB because the
injectors are bigger so I'm getting one from a scrapyard today is there any
numbers I can check to make sure it a 350 TB?
Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here.
All logos and trade names are the property of their respective owners.