GM 350 TBI Mystery Issue, Any help out here

[64osby]

I flipped engines and transmissions in my 92 Caprice wagon about 2 months ago; I used my other wagons engine, '93. It had sat for a year, but was running perfectly before the tree fell on it.

Ran ok for a while and now there is a stutter / stalling just above idle. I can also feel a slight surging when I'm cruising and the motor is not being pushed. It idles perfectly. If I slowly accelerate sometimes it stumbles other times it is fine. Quick acceleration gets it going with no issues.

Used the dist. carb, alternator and starter, from the engine I pulled, the '92 New cap, rotor, plug wires and plugs , timing is set correct. I have run some SeaFoam through it.

I finally had to increase the timing a little to keep it from stalling, but it will still stutter.

I took it to my shop, they have looked at it for a couple days and the only thing they say may be going on is the valves might be sticking?

Anyone have any advice on what my next step to solve the mystery issue.

 

[stackz]

Re: GM 350 TBI Mystery Issue, Any help out here

if its anything like the early 90's gm trucks...it probably has an engine computer that ends in 749. just because the part # is the same doesnt mean the calibration on the main chip is. a 749 series ecu was used on engines ranging from 3.8l v6's to the 454ss chevy truck.

end point, grandpa had a 89 1500 305. we swapped in a 91 350 setup, had to use the donor truck's ecu to get it to run right. other than that, do comp/leakdown and then try to run codes. if the donor set up for a time, sensors/injectors could have developed problems...

 

[64osby]

Re: GM 350 TBI Mystery Issue, Any help out here

Stackz - basically same engine as the trucks. I was wondering about flipping the cpu just to see what happens. It's a good thing I still have it.

No codes have popped on this even with it stalling.

 

[GA_Boater]

Re: GM 350 TBI Mystery Issue, Any help out here

Fuel pump or filter. Check the fuel pressure. Just a semi-educated guess.

 

[64osby]

Re: GM 350 TBI Mystery Issue, Any help out here

Fuel pump or filter. Check the fuel pressure. Just a semi-educated guess.

My shop checked fuel pressure and it was good, it was one of the things I had thought of also.

 

[j_martin]

Re: GM 350 TBI Mystery Issue, Any help out here

Peer down the throttle body and make sure both injectors are putting out a nice cone.

Been there on various vehicles.

 

[sschefer]

Re: GM 350 TBI Mystery Issue, Any help out here

Check for a vaccum line that's cracked or come loose. Don't forget that sneaky little bugger down at the tranny if it's an automatic. There's not a 350 around that likes vaccum leaks.

 

[dolluper]

Re: GM 350 TBI Mystery Issue, Any help out here

Change out the MAP to the 93's and check the disy for free play in the bushings could be sticking as for the increase to get it to run....did you disconnect the black {white or yellow}wire connector by heater fan motor to set base timing....usually sets code when disconnected 43? don't remember exact number

 

[64osby]

Re: GM 350 TBI Mystery Issue, Any help out here

I drove the wagon the last 3 days and it hasn't been stumbling or stuttering. The shop had tried a few things but siad they couldn't fix it, but now it seem to run pretty good, there really nice and didn't charge me anything.

I've been busy cutting / splitting wood and trying to get leaves up, so I did not take any time to look at the wagon

Peer down the throttle body and make sure both injectors are putting out a nice cone.

Been there on various vehicles.

I'll check the spray pattern.

 

[64osby]

Re: GM 350 TBI Mystery Issue, Any help out here

Check for a vaccum line that's cracked or come loose. Don't forget that sneaky little bugger down at the tranny if it's an automatic. There's not a 350 around that likes vaccum leaks.

I don't recall if there was a vac line to the tranny, 700R4. I remember 2 elec. /sensor plugs and fluid in and out lines. I'll have to take a look.

I did check the vac lines on top when I switched motors.

 

[64osby]

Re: GM 350 TBI Mystery Issue, Any help out here

Change out the MAP to the 93's and check the disy for free play in the bushings could be sticking as for the increase to get it to run....did you disconnect the black {white or yellow}wire connector by heater fan motor to set base timing....usually sets code when disconnected 43? don't remember exact number

I'll pull the MAP from the 93. Dist. is good. Fan motor disconnect??? Never heard of that. There is a wire that runs to the timing sensor, brown, on the top of the motor, passenger side that gets disconnected. Ref Chiltons

 

[j_martin]

Re: GM 350 TBI Mystery Issue, Any help out here

When the gremlin comes back, look down the TBI like I said.

 

[dolluper]

Re: GM 350 TBI Mystery Issue, Any help out here

Fan motor NO lol's it's the timming bypass wire runs close to fan motor...I guess you found it...shouldve thrown a code when disconnected.....if using a scanner check under emissions for cam offset should be within 1 degress + or - base

 

[robert graham]

Re: GM 350 TBI Mystery Issue, Any help out here

That carb has a bronze or paper fuel filter on the front/top behind those big hex nuts. Filter is small and can get stopped up easily...might check it. Might need to take top of carb off and check for water or trash in float chamber. Vacuum hoses are notorious for cracking and rotting...might check them all, especially that one that runs to the distributor. Check distributor vacuum advance which get cracks in diaphragms...easy/cheap to replace. Good Luck!

 

[j_martin]

Re: GM 350 TBI Mystery Issue, Any help out here

That carb has a bronze or paper fuel filter on the front/top behind those big hex nuts. Filter is small and can get stopped up easily...might check it. Might need to take top of carb off and check for water or trash in float chamber. Vacuum hoses are notorious for cracking and rotting...might check them all, especially that one that runs to the distributor. Check distributor vacuum advance which get cracks in diaphragms...easy/cheap to replace. Good Luck!

\

TBI -- Throttle Body Injection

Yer thinkin' on a 1970 quadrajet

 

[Wingnutt]

Re: GM 350 TBI Mystery Issue, Any help out here

The IAC (idle air control motor) is probably gummed up and sticking. Unfortunately, dis-assembly and cleaning is not an option. Read up on how to reset it here: http://www.thirdgen.org/techboard/tbi/ if that doesn't work you will have to replace it. There are some TBI gurus over there.

 

[robert graham]

Re: GM 350 TBI Mystery Issue, Any help out here

\

TBI -- Throttle Body Injection

Yer thinkin' on a 1970 quadrajet

In his first post he mentioned using the "Dist., Carb., Alternator and Starter from the engine I pulled"....I thought "Carb." meant carburetor....It must be either a Carb. or TBI, can't be both...What you think?

 

[j_martin]

Re: GM 350 TBI Mystery Issue, Any help out here

In his first post he mentioned using the "Dist., Carb., Alternator and Starter from the engine I pulled"....I thought "Carb." meant carburetor....It must be either a Carb. or TBI, can't be both...What you think?

Well, in 93 most everything was fuel injected, likely TBI. A throttle body sorta looks like a carb at first glance, so I just let it slide. He isn't claiming to be the expert after all.

 

[robert graham]

Re: GM 350 TBI Mystery Issue, Any help out here

Whatever induction it is, maybe it still has the old vacuum operated distributor advance that was notorious for cracked diaphragms and would cause his motor's symptoms?....worth a look-see!

 

[j_martin]

Re: GM 350 TBI Mystery Issue, Any help out here

Whatever induction it is, maybe it still has the old vacuum operated distributor advance that was notorious for cracked diaphragms and would cause his motor's symptoms?....worth a look-see!

Yer still stuck in the 60's. The same ECM that fires the injectors calculates the timing.