96 cobra breaks up under WOT

Pedrothecobra · Jan 20, 2022

96 cobra purchased two months ago. We figured out quickly it had a issue
MAIN ISSUE- car breaks up at 4K bad.
-Feels sounds like a misfire.
-No power at 4K (falls on its face)
-Crack throttle quick soon as it gets to 4K it breaks up.
-roll into throttle easy you can get it past 4K without it breaking up. That’s it tho any quick movement past 4K it does it!
THINGS WE DID:
1-Imrc delete (verified that is all 100%)
2-Plugs/wires/coils
3-throttle position sensor
4-fuel pump
5-all new gaskets (intake,valve ect)
6- new ecu (old had a chip and was just a mess)
7- all new O2 sensors
8- injectors cleaned and flow tested.. 100% ok
9- New MAF
10- vacuum system smoke tested and leaks repaired
11- new fuel regulator

Car gets over 4k now no problem if i ease into the throttle, but if i lay into it is when it breaks up. been getting better with everything i find and repair im kinda at a wall right now and just want to rule out the entire fuel system before i move on to the next system

so i found a bad splice at the inertia switch the other night, i repaired the wire. i now have what i believe is a good connection to the fuel pump. im getting 11.5V at the pump while at idle and 12.5V as the rpms go up. i have 13.5v on the body, im seeing 36lbs at idle on the rail and 50lbs when i remove the vacuum line. my question is should i be seeing more voltage at my pump? looking at the schematic i have it appears the ECU controls the fuel pump. but im not sure TIA

96blak54 · Jan 20, 2022

Im thinking the voltage at the pump should be whatever the alternator is putting out. Its not uncommon for a bad alternator or a diminishing one to cause problems. MAF's can be a major pain! Old or new.

Also what else is not uncommon are the timing wheels just behind the front timing chain cover are known to crack. They are made of powder metal and were eventually re-engineered into a stamped steel version. Any data logging should reveal what the timing is doing.

Pedrothecobra · Jan 20, 2022

so i have a pretty decent scan tool, im able to view most PIDs but unfortunately the crank sensor isnt one of them. ive been leaning towards just replacing the crank and cam sensors but i want to make sure it not still a fuel delivery issue first. my alternator is definitely putting out 13.5v but i find its strange i dont get that at the pump and just would like to know if i should see it there or not before i move on

D3VST8R96GT · Jan 20, 2022

Have you done a compression test? I think in the last thread there was a mention of one bank being extremely rich. one bank with cam timing out could cause issues. Wouldn't be my first guess but 30 mins of work to rule at this point isnt much. Is the fuel pump good?

We have to consolidate the threads.

Pedrothecobra · Jan 20, 2022

i have done a compression test. compression was 210 a cylinder. havent done a leak down yet. the fuel pump was replaced at a shop about a month ago but idk if i really trust it at this point. thats why im trying to figure out if the voltage i have at the pump is ok or not before i drop this tank to verify it was replaced. i pulled the plug from cylinder 5 last night to see how it looked after replacing the MAF and it actually looked like it was burning better then before and my LTFT were down around 0-5 as opposed to 15-20.

how can i consolidate the threads?

ttocs · Jan 20, 2022

I was waiting for the experts to chime in before I said anything since I have never actually taken voltage measurements of the pump. Its not uncommon to loose some voltage from the front to the back of the car on long runs but 11.5 is too low at idle and I didn't think its supposed to change with the throttle?

I have found that the harnesses going to my electrical accessories have started to wear out and the price to pay for that bad connection could cause this. You can test this by checking the voltage on both sides of the harnesses going to it(the inertia switch in the trunk, the harness on the bottom of the trunk that connects to the tank) but often by doing these tests and just dicking with the harnesses you will see the voltage change. I would probably put the test leads at the pump and then go and just grab/push/pull/dick with all the harnesses going to it and have someone watch to see if the voltage changes. When it does that is your harness.

Pedrothecobra · Jan 20, 2022

so i checked the voltage before the inertia switch originally and had 1v more then what i was getting out of it, thats how i found the bad splice some hack put in. are these pumps pulse width modulated or constant 12v?

ttocs · Jan 20, 2022

I am fairly sure they are 12v constant but I have not had stock stuff in my car for so long I would wait for a 2nd opinion. After you fixed the splice it gained that volt? It could be a bad switch, otherwise I would go under the car and be sure that the harness to the tank looks ok and is nice and tight.

Pedrothecobra · Jan 20, 2022

yea i gained the volt back

ttocs · Jan 20, 2022

I would still hope to be closer to the 13v area honestly. Are you running underdrive pullies?

Pedrothecobra · Jan 20, 2022

ttocs said:
I would still hope to be closer to the 13v area honestly. Are you running underdrive pullies?

no

ttocs · Jan 20, 2022

IIRC the fuel pump relay is in the CCRM, you might want to make sure its harness is still tight and looks good

Pedrothecobra · Jan 20, 2022

ttocs said:
IIRC the fuel pump relay is in the CCRM, you might want to make sure its harness is still tight and looks good

To I have checked the connection to the Ccrm already. Someone thought it was a good idea to try and bypass the AC pressure switch. I haven’t cracked the actual unit open but I did check the connection while I was removing and repairing the harness. What’s IIRC?

ttocs · Jan 20, 2022

if I remember correctly.

Check the voltage out for the pump at the ccrm and see if its different from at the inertia switch then. If its low there then you might need to have the ccrm checked out if it has a higher voltage then your drop is somewhere in between the ccrm and the inertia switch

Pedrothecobra · Jan 20, 2022

ttocs said:
if I remember correctly.

Check the voltage out for the pump at the ccrm and see if its different from at the inertia switch then. If its low there then you might need to have the ccrm checked out if it has a higher voltage then your drop is somewhere in between the ccrm and the inertia switch

Is this system supposed to have a constant 12v? Or should it vary depending on demand?

96blak54 · Jan 21, 2022

Should be whatever the main voltage is.

Pedrothecobra · Jan 21, 2022

96blak54 said:
Should be whatever the main voltage is.

ok thats what i thought just wanted to verify. i drove the car to work this morning with the pressure gauge on the rail and i am dropping pressure when i get on it. it drops to about 20-25psi. and my voltage is under 12v. ill start traceing down the wires at the CCRM to the fuel pump

ttocs · Jan 21, 2022

what does the alternator put out for voltage?

Pedrothecobra · Jan 21, 2022

ttocs said:
what does the alternator put out for voltage?

13.5

ttocs · Jan 21, 2022

ok so the voltage is there, its just getting lost somewhere. While your checking the fuel pump power at the ccrm see if you can find the power wires for it and see what its voltage is. If the ccrm is only getting 11v, it can't sent the pump 13.