Hot Rod Forum banner
1 - 2 of 2 Posts

· Registered
Joined
·
16,303 Posts
So as the title states i have a 1989 C1500 Scottsdale with a 350 TBI TH700r4. I belive it has the stock gears though i was told it has 3.73. Anyhow there are a couple different things.
First it has to crank 4-5 times before it will fire on a cold start, Hot start it will fire up with 1/2 to a full crank. I cant find any vacuume leaks. Starter was tested and is okay. All grounds are fine(dunno if that would even affect this). I have checked pretty much everything i can think to check except the TPS.

Second of all when going down the road when i give it throttle to speed up it kinda bogs down, i have to pretty much stand on it so it will rev up( come out of OD) and gain speed. It doesnt matter if its in OD though. Does the same in D, i cant gain speed unless i make it downshift otherwise it boggs. It idles fine. Runs a little rich i can smell the gas in the exhaust but i also dont have a Cat.

Here is a Ful;l list of everything that has been replaced in the last 6 months. Fuel pump, fuel filter,(oil n filter), pcv, plugs, wires,Dist(complete with cap and rotor), Ignition coil(ACDelco), Rebuilt the TBI yesterday, New injectors, CTS, MAP sensor, EGR valve, reset the timing with a light( just put it at stock 0* BTDC). I cleaned but didnt replace the IAC when i rebuilt the TBI it was pretty dirty but seems to work properly.

I cant for the life of me figure out what is wrong. It idles perfect great throttle response. Just boggs when doing about 40-60 and i try to give it gas like to go up a hill. I have checked everything 100 times to no avail. I do get a soft code from time to time after driving for a while but since its a soft code and i dont have an OBD1 scanner i dont know what code it is throwing ( i know no help there):nono: The only parts i havnt replaced or thought to really check until recently are the O2 which is a $15 fix and the TPS which run about $55 here. i dont know if wither would cause the problems i am having.
**Note** i have an adjustable fuel pressure regulator and when i redont the TBI i raised it 2 full turns. Seemed to help a little on the low end but not where i need it. Also i do not have a Tach or Pressure gauge. Not that it matters one way or the other but it will still burn the tires off, but only if i hold the brake(doesnt like to "peel out"). Sorry for the Long post.
The startup doesn't sound that unusual for a cold start but begs the question how cold? TBI suffers the same problems in the plenum and ports as do carbureted engines but it has even less heat as the intake is water cooled or heated in this case. What looks like an exhaust cross over isn't the driver side goes nowhere the passenger side feeds the EGR so the intake doesn't get hot till the coolant does. The manifold still carbons up to a point where it doesn't supply enough exhaust to keep the EGR functional in terms of the amount of exhaust gasses available to the intake, this will set code 32. Additionally, cruise RPMs is where EGR is functioning, without enough exhaust gas the mixture falls lean because the intake is filling that space with air. If there is enough space in the computer logic and the O2 sensor is good it will add fuel whether enough to correct the mixture I don't know. Code 12 is the computer telling you it's alive and well. Code 22 could be a mixture discrepancy between the throttle position and the O2 data. OBD-I isn’t too discrete; the diagnostics require the tester to have an understanding of systems logical relationships rather than having the ability of the diagnostics to point and grunt "go here, fix problem".

A 1989 brings pictures of high mileage; high mileage brings pictures of a dying timing set. Chain stretch causes the cam to lag the crank which also takes the distributor with it. In the case of TBI the computer adjusts the timing which if this was a carb you'd be doing it and might become aware that constant distributor adjustment was an indicator that something was wearing out, in this case that's transparent till it gets beyond what the computer can patch up. Idle could be right but as engine RPMs pick up the computer may run out of authority to fix the problem. From that point on the spark would appear to be late this would clip further power increase is some amount if not total. With the brown/black wire disconnected so the computer isn’t playing with the timing run the RPMs up about 3000 then slam the throttle closed, with a timing light on the marks, if they move off and around the zero mark, you know the cam ain’t tracking the crank or the distributor is out to lunch in terms of a worn gear and/or bearings.


Bogie
 

· Registered
Joined
·
16,303 Posts
Several years ago I had an 89 that came in to be checked out for a very similar problem.
Took some looking to find it, but....and I am not saying yours will be the same but....I found the wire to the pickup coil in the distributor were corroded almost in two. I replaced it and it must have been the problem because it fixed it.
Funny thing is it still had spark but wouldn't spray the injectors.
Must have been sending a signal that was too weak to Feed the Ignition and the ECM
That being said, the TBI system is very good at startup especially cold as it sprays the fuel, promoting atomization, unlike a carb that requires the fuel be siphoned through small orifices in the carb before it even atomizes .
A TBI is a very dependable cold starting system even in the face of -25 degrees and better.
I wouldn't worry about any timing chain problems at this point as it is a dual row chain and not the morse silent type. The dual row is used in roller cam engines because it is less prone to allow timing fluctuations due to design alone.
I would try spraying down the throttle and then try to start it first thing in the morning to see what you get.If it starts with a spray then fuel is lacking for some reason.
It would really benefit the diagnostic routine if you had a pressure gauge on the system to verify cold startup pressure or lack of, or slow to build pressure.
Better to diagnose than to throw parts at it.


One other option for cold startup fuel starvation (a presumptive test) would be to turn the key on don't crank, wait 2 seconds. This cycles the fuel pump by using the ECM 2 second pulse that turns the fuel pump on to prime the system. You can cycle the key 4-5 times. Be sure to listen to be sure the fuel pump relay is indeed cycling. Then after that see if you get a decent cold startup.
If that seems to help the startup situation the it is likely the pump check valve is not holding or the line in the tank between the pump and the fuel outlet line is leaking.


At any rate you really need to nail down if your cold start problem is fuel related.
Be sure the fuel pump relay will cycle and that it is powering the circuit
Get a fuel gauge and hook it up.
Stop guessing at it.
The factory did not use a roller chain in flat tappet cammed engines and did not use a double row roller in roller tappet engines it was single row roller which does have a bit better life than the morse chain over a plastic tooth gear b ut not near the life of a double row roller chain.

Bogie
 
1 - 2 of 2 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top