Motor Swap Problem/Suggestions

JimHTP

Original poster
Member
Jul 30, 2012
27
Hello everyone -

My father and I have been working on my 2004 chevy trailblazer with the 4.2 i6. Last year around emissions time, chasing down codes led us to find out there was excessive crankshaft end-play , and we secured a used good 4.2 to swap in. Many hours and weekends later, the engine is in but we are having some problems. The replacement engine came with a PCM, which while on would not get the starter to turn. We swapped it to the PCM from the old motor, and voila the starter will turn the engine over...but the truck won't catch and start. We are at a loss now as to what to try next. We have checked all connections and grounds and everything seems to be ok.

The fuel pump does not seem to come on at all when turning the key or attempting to start. I'd think I would be able to feel or hear the pump going from under the vehicle, and I can't.

I plan to pull down the gas tank this weekend but I'm fairly certain the problem isn't there. Nothing else so far has been that simple. Anybody have ideas, suggestions, checklists, ANYTHING?


I started this thread on another forum and have already gotten some very helpful replies like: jumper the fuel pump really and check the injectors and coils for power to start narrowing the problem down. But I'd really like to get as many suggestions from knowledgeable trailblazer owners as possible here. I'm tired of screwing with this thing and carpooling/begging/borrowing rides to work!
 

anthonyl79

Member
Jul 15, 2012
127
Colorado
What year of did the donor motor come out of? I did a motor swap a year ago and the only problems I had where was the cam solonoid was bad and needed to to do a crank re-learn on it. If it came out of the same year then I would be looking at some possible wiring that got left off, I know there are several grounds down on the drivers side just above the oil pan. Just above that there are several connectors to control who knows what. Good luck and let us know what you come up with.
 

JimHTP

Original poster
Member
Jul 30, 2012
27
Same year according to the salvage yard, hope they didn't lie!

I think I got all those grounds. They made me sweat worrying about this kind of thing so i carefully labeled them all. That compartment is so tight and wires so tangled and numerous it would be so damn easy to miss though.

Thanks for the suggestions!

Was it as big a pain in your ass as it was mine to do this motor swap? Hands down worst one I've ever done.
 

anthonyl79

Member
Jul 15, 2012
127
Colorado
JimHTP said:
Same year according to the salvage yard, hope they didn't lie!

I think I got all those grounds. They made me sweat worrying about this kind of thing so i carefully labeled them all. That compartment is so tight and wires so tangled and numerous it would be so damn easy to miss though.

Thanks for the suggestions!

Was it as big a pain in your ass as it was mine to do this motor swap? Hands down worst one I've ever done.

Yes it was a pain in the ass! Luckly I did mine at my friends shop. So I had a lift. He helped me stab the new one in and he said if one ever comes through his door there is no way he will take it on. How did you like the 4 x 4 portion of the swap?

I would not see a reason you would need to swap the PCM unless somehow the original got damaged.
 

JimHTP

Original poster
Member
Jul 30, 2012
27
Lucky you, I did it in the front yard!

Didn't have to mess with 4x4, 2wd thank god. Kinda crazy that motor still takes 7qts of oil with half the oil pain cut out for axles.

The pcm attached to the new motor is the one that wouldn't work, my original recognizes the key or vice-versa and turns the starter just fine.
 

anthonyl79

Member
Jul 15, 2012
127
Colorado
JimHTP said:
Lucky you, I did it in the front yard!

Didn't have to mess with 4x4, 2wd thank god. Kinda crazy that motor still takes 7qts of oil with half the oil pain cut out for axles.

The pcm attached to the new motor is the one that wouldn't work, my original recognizes the key or vice-versa and turns the starter just fine.

As far a I know you would need to use the original one due to the security features without having to relearn it. I would look over all the fuses and what not. Wonder if something got shorted out, or something maybe got plugged in to the wrong spot by accident.
 

MAY03LT

Member
Nov 18, 2011
3,420
Delmarva
anthonyl79 said:
....a crank re-learn on it.

:iagree: That will have to done.

Have you tried alternate fuel? Like spray a shot of either into the intake and see if she runs for a second?
 

JimHTP

Original poster
Member
Jul 30, 2012
27
MAY03LT said:
:iagree: That will have to done.

Have you tried alternate fuel? Like spray a shot of either into the intake and see if she runs for a second?


Nope, another mechanic friend just suggested this so I will try this weekend.

I have a brand spanking new CPAS as I tried that before I figured out my last motor had been crank-walked. I'll swap that in before I get the relearn done.

A friend who runs a mechanics shop in town said he could do this by following some procedure involving bouncing it off the rev-limiter and turning it off in a certain sequence. I tried to do it myself using a method that involved driving at specific speeds for specific distances, but of course I'll never know if it worked since that wasn't the problem. Do you know of any of these methods for the relearn?

Thanks for the helpful suggestions so far everyone!
 

anthonyl79

Member
Jul 15, 2012
127
Colorado
JimHTP said:
Nope, another mechanic friend just suggested this so I will try this weekend.

I have a brand spanking new CPAS as I tried that before I figured out my last motor had been crank-walked. I'll swap that in before I get the relearn done.

A friend who runs a mechanics shop in town said he could do this by following some procedure involving bouncing it off the rev-limiter and turning it off in a certain sequence. I tried to do it myself using a method that involved driving at specific speeds for specific distances, but of course I'll never know if it worked since that wasn't the problem. Do you know of any of these methods for the relearn?

Thanks for the helpful suggestions so far everyone!

I believe a scanner is the only way. But it has to run to do the relearn. Mine started and ran just had codes and ran likd crap.
 

MAY03LT

Member
Nov 18, 2011
3,420
Delmarva
JimHTP said:
Do you know of any of these methods for the relearn?

I remember reading a procedure that someone posted on trailvoy when someone else was trying to do it after getting a tuned pcm. They couldn't get it to work and I don't recall reading about anyone who was successful. Like anthony said, it should run (probably crappy and have a CEL) so I'm thinking the problem is elsewhere.

Other things you can do/check is verify fuel pressure (with a gauge), check for spark, and check for cranking vacuum.
 

limequat

Member
Dec 8, 2011
520
Unless you have reason to believe that there is something wrong with it, you should probably be using your stock PCM. That way you don't have to worry about VATS.

With original PCM, check basics. Got fuel pressure? how much? Pull out a plug/coil. Got spark?
 

JimHTP

Original poster
Member
Jul 30, 2012
27
Lot's of great ideas guys. It was also suggested that putting the wrong pcm in may have screwed up the bcm/pcm interface and I'll need to do a relearn.

Brief summary of what I'm thinking I will try this weekend, in order to try and fix this

PCM/BCM relearn
Starter fluid/carbuerator cleaner in the TB while trying to start to see if it cranks
Jump fuel pump relay and try to start
Check power at coil/plu
Check power at injectors
Check power somewhere upstream of the fuel pump
Jump off a bridge

Anybody here know of any links to the BCM reset procedure?

Thanks everyone for all your help!
 

jimmyjam

Member
Nov 18, 2011
1,634
As stated, start with the basics. I doubt the bcm is causing you any issues. It wouldn't affect the fuel or spark. Start there and do yourself a favor don't start digging further than you have to
 

limequat

Member
Dec 8, 2011
520
jimmyjam said:
As stated, start with the basics. I doubt the bcm is causing you any issues. It wouldn't affect the fuel or spark. Start there and do yourself a favor don't start digging further than you have to

Agreed.

There is a "security relearn" procedure, but it has to be deliberately invoked. It won't happen on its own.
 

bravad'oh

Member
Sep 15, 2012
44
What about the ignition key? Is it not linked to the PCM? Maybe your PCM is looking for the key that was in the donor vehicle.
 

Wooluf1952

Member
Nov 20, 2011
2,663
Milwaukee, Wisconsin
bravad'oh;95102 said:
What about the ignition key? Is it not linked to the PCM? Maybe your PCM is looking for the key that was in the donor vehicle.

Our trucks don't have chips in the key. As long the key turns to the start position, the PCM doesn't look any farther.
 

bravad'oh

Member
Sep 15, 2012
44
Wooluf1952 said:
Our trucks don't have chips in the key. As long the key turns to the start position, the PCM doesn't look any farther.

Right. We'll have to wait until he checks the basics and reports.
 

dfc739

Member
Jul 29, 2012
170
Des Moines, IA
I swapped my motor a couple weeks ago over the Labor day weekend. We used the bad motor's PCM and left all the sensors that came in the replacement motor. We did not do the crank relearn procedure and it still hasn't thrown a code for it. Maybe I got lucky and the bad and replacement motors were so close that it didn't matter? Just my :twocents:

And yeah, the 4x4 going through the oil pan was a great idea. It only took an additional five or six hours because of it (removing axles, removing front differential from oil pan, reinstalling).
 

anthonyl79

Member
Jul 15, 2012
127
Colorado
dfc739 said:
I swapped my motor a couple weeks ago over the Labor day weekend. We used the bad motor's PCM and left all the sensors that came in the replacement motor. We did not do the crank relearn procedure and it still hasn't thrown a code for it. Maybe I got lucky and the bad and replacement motors were so close that it didn't matter? Just my :twocents:

And yeah, the 4x4 going through the oil pan was a great idea. It only took an additional five or six hours because of it (removing axles, removing front differential from oil pan, reinstalling).

Someone correct me if I'm wrong here, but you did not need to do the crank relearn since you used the PCM that came with the motor. When you start changing out the CAS, and PCM that is when you need to relearn the PCM to the new CAS. When I did my swap I used the donor motor's sensors and that is when I needed to do the relearn. And gotta love the axel through the oil pan. Big :thumbsup: to GM on that one.
 

Mooseman

Moderator
Dec 4, 2011
25,335
Ottawa, ON
Whenever either one of these is changed or disturbed, you need the crank angle sensor relearn:
Crank sensor, cam sensor, PCM, complete engine.

Crank to cam angles are unique to each individual engine so the relearn is required. It will NOT prevent you from starting the engine. It's only for the error.
 

dfc739

Member
Jul 29, 2012
170
Des Moines, IA
anthonyl79 said:
Someone correct me if I'm wrong here, but you did not need to do the crank relearn since you used the PCM that came with the motor. When you start changing out the CAS, and PCM that is when you need to relearn the PCM to the new CAS. When I did my swap I used the donor motor's sensors and that is when I needed to do the relearn.

I'm not sure why, but we didn't have to do the crank relearn. We used the original PCM from the motor that went bad. It was the PCM already in my truck. The replacement motor did not come with a PCM. So the two engines must have had a close enough crank angle that the PCM from the old motor could be used for the replacement motor. Or it's about to pop a code any minute... knock on wood.

anthonyl79 said:
And gotta love the axel through the oil pan. Big :thumbsup: to GM on that one.

I'm raising a finger at GM, but it's not my thumb...
 

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