1991 Gm Engine Swap gone wrong???

[s4s54]

I have a 91 chevy k1500 w/350 engine that I did an engine swap on last year. Since this is not a daily driver it has been low on the priority list to find the problem. Here's what happened: Went to salvage yard and bought a "350" out of a 91 caprice wagon. (My fault that I did not do more research. I turned out to be a 305 )

Well it took me 6 months to get to putting the motor in soo.. returning the motor was not an option.

 

After swapping the motors everything was good. Truck ran fine, decent power etc.. I though I had gotten lucky. Then it started to get cold in the mornings and would not start. (Loading up I presume). I started replacing Parts: the intake temp sender (x3), Throttle pos. sensor, map sensor, O2 sensor, EGR Valve, ESC box, Different Distributor etc...(X2) Nothing seemed to make it run better. So I bought an ECM from the local salvage parts yard. No change, It was a 350 computer though and the engine is a 305.

 

Here's where I am today: The truck will run and start even when it is cold as long as the plug in on the firewall (the one that advances the dist) is disconected. the base line timing is zero and holds rock steady there. Revs up and runs very nice even while test driving it. Plug it in and restart and the timing is all over the place. (up to 37 deg at idle!) the truck will barely start and it blows black smoke and shoots 6' flames out the tailpipe.

 

Question 1 Is there a difference between the 305 ande 350 computers when it comes to spark curves? ( I have tried 3 different 350 computers and the prom chip out of an 88 305) How about fuel delivery? (It has the 305 tbi and injectors.)

 

I have replaced just about everything I can think of.

 

Question 2 Is there anything I am missing here??

 

 

Any info would be appreciated

Thanks

Duane

[95Sierra2500]

Sounds like your timing sensor or that plug you're talking about is fubar'd. Replace that and see what happens.

[s4s54]

Found the problem, Turned out to be a broken ground wire. Seems to bre running well.

[Corky356]

Found the problem, Turned out to be a broken ground wire. Seems to bre running well.

 

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Glad it's running well, but which ground wire was broken and messing up your timing?

[s4s54]

There are two pirs of ground wires that go to the engine block up by the coolant temp sensor. When I installed the new engine, I broke the eyelet on one ot the pairs. I twisted the wires together and sandwiched them between two nuts on the thermostat housing.

First mistake was not going to the store and replacing the wire connector when I put the motor in.

Second mistake was mounting them to the thermostat housing. (I was told that this is a very bad place to use as a ground point.)

Third mistake was not checking this before spending a ton of money on sensors, ecm's, and distributors.

 

To elaborate on you question however, the two wires are brown in color and both are terminated into the same wire connector. I was told to make sure that you ground both of these pairs to the same bolt that is being used to ground the neg cable for the battery to the block.

Works great now. I am even noticing much better fuel economy because the computer actually knows what is going on. (apparently the computer needs those wires to be grounded to controll the spark and the fuel curves).

 

This is how it was described to me by the guy who helped me fix it. If I am wrong tell me now, but I cant argue with him because it runs great.

 

Later

Duane

[Corky356]

There are two pirs of ground wires that go to the engine block up by the coolant temp sensor.  When I installed the new engine, I broke the eyelet on one ot the pairs.  I twisted the wires together and sandwiched them between two nuts on the thermostat housing. 

First mistake was not going to the store and replacing the wire connector when I put the motor in.

Second mistake was mounting them to the thermostat housing. (I was told that this is a very bad place to use as a ground point.)

Third mistake was not checking this before spending a ton of money on sensors, ecm's, and distributors.

 

To elaborate on you question however, the two wires are brown in color and both are terminated into the same wire connector.  I was told to make sure that you ground both of these pairs to the same bolt that is being used to ground the neg cable for the battery to the block. 

Works great now.  I am even noticing much better fuel economy because the computer actually knows what is going on.  (apparently the computer needs those wires to be grounded to controll the spark and the fuel curves). 

 

This is how it was described to me by the guy who helped me fix it.  If I am wrong tell me now, but I cant argue with him because it runs great.

 

Later

Duane

 

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Sounds good to me.