Crazy engine surging after new short block, help needed

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hisandhers80s

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Madison, MS
1994 FZJ80 - I installed an all new short block, but had the head machined, cleaned, etc. After getting it all back together, and running, at 3 minutes it goes into this engine surge from 800 RPM up to almost 3,000 RPMs and fluctuates like that until I shut the engine off. I have solved the power steering whine you hear in the video. Here is everything I have tried to solve this issue:

Replaced IAC with known good IAC
Recalibrated the TPS
Replace the TPS with a known good one and recalibrated
Smoke tested the engine and found zero vacuum leaks (tested multiple times)
Replaced the AFM/VAF (twice just to are sure I had a good one)
Tried disconnecting the O2 sensors
Changed and messed with the timing
Adjusted Throttle, kick down, and cruise cables, and tried disconnecting entirely from the throttle body

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Coolant temp sensor can do weird stuff to the idle. So it’s idling good on initial start up then going into varying rpm after a couple minutes?
 
Yes at about 2.5-3 minuets of running. I have gotten it to run long enough to get the thermostat to open and circulate the coolant yet.
 
I am sorry to hear this! I am dealing with pretty much the same thing. We will scratch our heads together lol. I am going to try the coolant temp sensor tomorrow and if that helps me I will let you know.
 
I am almost certain I put a new coolant temp sensor in the short block, but I will check my records. Another potential factor is the engine wiring harness. I replaced every plug connector on the entire harness. I had a misfire at first and found one of the injector plugs did not accept the pins i had put in the plug. I was thinking that maybe there is another plug somewhere on the harness that is doing the same thing. I was thinking about pulling the entire harness again and double checking everything to see if there is something that I missed or messed up, but really did not want to have to do that.
 
hisandhers80s said:
I am almost certain I put a new coolant temp sensor in the short block, but I will check my records. Another potential factor is the engine wiring harness. I replaced every plug connector on the entire harness. I had a misfire at first and found one of the injector plugs did not accept the pins i had put in the plug. I was thinking that maybe there is another plug somewhere on the harness that is doing the same thing. I was thinking about pulling the entire harness again and double checking everything to see if there is something that I missed or messed up, but really did not want to have to do that.
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Try unplugging one connector at a time start with the injectors, and see if the condition changes, if it doesn’t further investigate that one, if it does change it’s likely good. There is two temp sensors one for the gauge and for the ECM that’s the one you want to check.

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Also what ever happened with your ECM injector pin out? I remember another post saying you were missing a wire or something which would be an anomaly.
 
So that was related to the engine misfire. It turned out that it was at the injector plug with the pin pushing out of the connector when I plugged it into injector # 5. I was able to re-pin the wire and get it in the connector and the misfire went away.

I also did purchase a new coolant sensor, so I think the plan will be to check the connector tonight and see if the pin pushed out of the connector when I plugged it into the sensor, and then I can also put the old sensor back in to see if that makes a difference as well.
 
While the engine is running good within that first 3 minutes before the surging starts unplug the IAC. If you still have the surge then the IAC/wiring most likely can be eliminated. Do you have a hand throttle? If so set it to 1200 rpm and see what happens after the 3 minute period.
 
ppc said:
While the engine is running good within that first 3 minutes before the surging starts unplug the IAC. If you still have the surge then the IAC/wiring most likely can be eliminated. Do you have a hand throttle? If so set it to 1200 rpm and see what happens after the 3 minute period.
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I have tried unplugging the IAC and it still goes into the surging if it is plugged in or unplugged.
 
CJF said:
Again: I don't see how 3K RPM is even possible if the throttle plate is fully closed.
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I tested that by having all of the cables (Throttle, Cruise, & Kickdown) unhooked from the throttle body. I do not know what would be opening the plate if there are no cables connected and it still happens. Last night it was only surging to 2100 RPMs after changing to the 3rd VAF/AFM. That is the lowest surge I have had still it started.
 
Time for the old brake clean spray to check for vacuum leaks? Either that or a smoke machine if you want to be more professional.

Either way I hope you figure it out! Gotta be frustrating.
 
Mag71k5 said:
Time for the old brake clean spray to check for vacuum leaks? Either that or a smoke machine if you want to be more professional.

Either way I hope you figure it out! Gotta be frustrating.
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I have a smoke machine I got on Amazon, and I have smoke-tested it twice and cannot find any sign of smoke. Even turned the lights off and closed the garage so it was pitch black and used a flashlight, and there was no sign of smoke.
 
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