Drivability specialists..I need help!
 

Thanks for clicking on the thread. This is my first post on this discussion board. I've been here a couple of times for advice on my 1990 3/4 ton Sub 7.4L TBI. Truck is all stock except for a performance chip.

This has been an ongoing issue since I bought the truck about 2 years ago. When I start the truck cold, it runs very rough and backfires through the exhaust. The longer it sits, the worse it gets (one week is worse than overnight).

I've also noticed that it has really poor mid throttle performance even after is warms up. The idle is a little high, but anything from 10% to about 90% throttle feels like it's running lean.

Here is the list of parts I've put on over the last 2 years:

Complete intake reseal
MAP sensor
TPS
O2 sensor
CTS
TWC
Spark plugs, cap, rotor, wires
ECM (swapped CAL pac chip from old)
Added a performance chip
Fuel pump and filters

I've checked, double checked, and triple checked the ignition timing and fuel pressure. The injector spray looks good. Compression is good warm (haven't checked it cold). Old changed regularly and motor is very clean on the inside. I've had a lot of experience on OBD II cars, but not so much on OBD I cars. I do have access to a scan tool and the integrator is 133 last I checked (running lean, commanding rich). I've disconnected the EGR valve and it still feels like it's running lean (a trick I learned from when I worked at Saturn). I haven't done anything with the throttle body. My next thoughts where to replace the pressure regulator and injectors possibly

Any ideas? I remember my '92 with pretty much the same drivetrain had a similar issue without the backfiring cold start. I installed the complete Edlebrock multi point conversion on it with the same issue (feels like it's running lean)

Oh yeah, one more thing, it has always passed CA emissions testing.

Hope I can get some help. Thanks again for any possible help. I look forward to sharing my knowledge with the board whenever possible

Re: Drivability specialists..I need help!
 

check to see if you have a vacuum leak. the throttle shafts on the tbi unit tend to wear out and will cause a vacuum leak

Re: Drivability specialists..I need help!
 

X2. Very common on high mile TBI. A reseal of the TBI won't fix it.

Re: Drivability specialists..I need help!
 

did you spray carb cleaner around the throttle body gasket to check for a vacuum leak

Re: Drivability specialists..I need help!
 

This may help.

http://67-72chevytrucks.com/vboard/s...d.php?t=445487

Re: Drivability specialists..I need help!
 

longer it sits,, the tbi shaft wouldn't change..
but a sticking valve would,, dump a qt of marvals in the oil..
it show lean if the charge is going right out the exhaust valve, and when fire'n you get a backfire.. if you're lucky you didn't burn the valve yet

Re: Drivability specialists..I need help!
 

Re: Drivability specialists..I need help!
 

Back firing through TB indicates that ignition timing is off or you have a valve opening at a wrong time. On high millage engines OE timing chain is known to stretch causing valve timing to be inconsistent. During 70's and early 80's GM used to install plastic cam/crank gears in a stupid attempt keep engine noise down. I have seen timing chain jump a tooth or two resulting in wrong camshaft timing vs. piston travel! Since you have replaced every sensor on your TB it looks like it might be engine mechanical and not EFI related issue.

Check your harmonic balancer outer diameter ring - high millage balancers are known to slip exterior ring (due to deteriorating rubber) resulting in incorrect ignition timing indication.

Run a compression (or better cylinder leak down test) check - if there is a stuck (or burnt) valve that will catch that easily before doing anything else.

//RF