Check engine light comes on only on compression, ( like down hill ) what could cause this???

 

Compression? Do you mean engine braking, where you keep it in gear but let off the gas?

I'm guessing it has no codes to pull after you get the CEL. What were the recent mods to the car, or is it something you just bought? Also, what type of car is it?

 

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The car is a 93 civic ex D16Z6 vetech, it is a built bock, with stock head,t3/t4 turbo 6psi street tuned, but no problems before tranny swap, i had just changed the tranny, and after getting it in, i had taken it on a test drive, (breaking in clutch) up hills fine only when "engine braking" (in gear no gas) down hills for just after like 5secs light comes on?? i did break sensor under the distibutor that gose into the head (but not the long skinny one but the one more towards the front of the car, just a round plug 2 wires, i wired it back together though? what is that for anyway? ), thats the only thing that went wrong putting the tranny back in????? may be something else????? thank you

 

what code are you getting. my bet is on the speed sensor, it will throw a code on decel in gear but not in neutral or any other time.

 

That will be your water temp sensor. Check the cell and see what it is.