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After motor change wont idle

(1991 Mitsubishi 3000GT SL)
in Flat Lick, KY on June 18, 2011
I changed the motor in a 3000gt the other motor started and ran just had a knock when it got warm. Replaced the motor and when u start it u have hold the accelerator to the floor to get it to started, then u have to hold it above 1500-2000 rpms to get it to run. It doesnt feel like it is creating any vacuum, and when u drive it, it feels like a pig very sluggish like it has no power. I have changed every sensor on the motor, including the IAC, and injectors. Mass air looks good on the scanner, What am I missing?
Displaying 2 answers & 9 comments
Popular Answer
on June 18, 2011
Was the engine an exact bolt in swap - identical year and model? How much of the old engine did you swap over to the replacement one?

Since everything was operating, the failure must be related to something incompatible or something mis-installed or not fully installed.

The last thought was a possible exhaust restriction, but I can't figure why if the old engine was fine.
on June 21, 2011
Yes the engine was an exact replacement. I have changed everything but the block and heads literally. The intake gets super hot hot when it runs even gets hot when it cranks. I had to cut the converter off of it because it got cherry red. It acts like it is lean. I would change the fuel pump but wouldnt a pump allow it to idle and not rev not vice versa. I am leaning towards the ecm but it worked ok with the other motor. i checked the timing and everything is fine there also. I have yet to check compression values.
on June 21, 2011
If your converter got cherry red, then it's got excess fuel, meaning that it's not lean at all; it's excessively rich.

It could be that the processor thinks that it's lean, and tries to richen up the fuel significantly, as with an air intake leak, vacuum leak at the intake, etc.

Since you have to hold the pedal to the floor to start, and you have this rich running condition - have you considered an injector or injectors stuck open?
on June 22, 2011
I have changed the intake and checked for vacuum leaks. I have changed all the injectors and fuel rail with the same results. There is nothing on the motor that has not been changed or checked. I have no vacuum to the brake booster, the brakes worked fine with the other motor. If I rev it up i get a slight feel of pedal like the vacuum is trying to get there. I also changed the throttle body and plenum. This thing is driving me batty.
on June 22, 2011
The vacuum is obviously where to focus your attention, because if you're not generating vacuum for your brake booster -- you have a huge clue there.

Check the vacuum canister, and the purge control vacuum solenoid valve. Something is not checking out...
on June 23, 2011
now i am occasionally get a pop back through the air box when it tries to start. checked purge valve and vacuum lines as well as canister no problems. could the cams b wore
on June 23, 2011
Exhaust manifolds have any leakage?

Pop through the air box still sounds like an air leak somewhere...

I have been assuming all along that the engine internally is acceptable...
on June 25, 2011
i am also assuming that the engine is mechanically sound. It doesnt seem to have an exhaust leak anywhere
on June 25, 2011
OK...maybe the next step is the compression test and leak down test if you find anything low...
on July 01, 2011
Finally done compression check and found I had no compression on frt cylinders. Removed valve cover and all of the rocker arms on one cam were laying in the bottom of the head. Motor never made a noise. Just assumed the motor was sound.
on July 01, 2011
Nice job. Wow, huh? Amazing what you'll find. Glad you got it resolved!
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