Will not stay running!!

[DroopYeyeS]

Ok boys and girls, I'm hunting needles at this point. I have just replaced the caps on the main board of my ECU. Upon first try, after the rebuild, the car started right up. It ran for about 20 sec then shut off. After it sat for about 2min, it started right back up, ran for 20 sec, and died. I've check the MPI relay as well, but it seems to be working. Any ideas on this issue?

 

[gofer]

Have you checked spark on all (4) cylinders or fuel pressure to the rail?

 

[Voodoo262]

What kind of fuel pump are you running, and is it rewired?

 

[DSMReviver]

Ok boys and girls, I'm hunting needles at this point. I have just replaced the caps on the main board of my ECU. Upon first try, after the rebuild, the car started right up. It ran for about 20 sec then shut off. After it sat for about 2min, it started right back up, ran for 20 sec, and died. I've check the MPI relay as well, but it seems to be working. Any ideas on this issue?

"replaced the caps on the main board" i don't know what that means but you might wanna check there first .

this might also be the source but not limited to the problem it self.

Good luck.

 

[Voodoo262]

"replaced the caps on the main board" i don't know what that means but you might wanna check there first .

this might also be the source but not limited to the problem it self.

Good luck.

This means that he replaced the capacitors on the circuit board in the ECU, as they're notorious for leaking after a number of years of service and causing bizarre problems.

 

[DroopYeyeS]

Exactly VooDoo. Plugs are fine, pressure is fine ( per the gauge on my AFPR). Seems like the MPI relay is not being energized continuosly. I've been looking through my FSM for the ciruit to see if perhaps another relay is involved but have not found any. Very upsetting, .

 

[1AWD90]

What do you lose when it stops running fuel or spark?

 

[gofer]

I think your chasing your tail on this one, it seems to me like you inserted the problem when you put that ECU in that had the CAPs replaced on it. Who knows how well of a job the tech did when they soldered them on there? Is there anyway you R&R your ECU with a working 91-94 ECU to eliminate the odds of the repaired ECU being the culprit?

 

[DroopYeyeS]

The ECU is one that I bought from a local parts store about 2yrs ago. The flexplate broke so the car has been down while I fixed that and made other changes. Upon putting it all back together, I started it up just to hear the infamous MPI relay on/off clicking. After some testing and searching, I pulled the ECU, opened it up, and found the caps had once again leaked out. I replaced the caps, not a tech somewhere. The car will now start, but dies after about 20sec. I used the caps outlined to be upgrades on the VFAQ (47uf 50v, 22uf 50v, 100uf 25v), but the seemed small compared to what is stock on the ecu. I'm wondering if the "upgrade" of 100uf 25v from the stock 100uf 16v, could be my issue. Also the stock 47uf 50v was almost twice the diameter of the new 47uf 50v. Any thoughts?

 

[gofer]

I don't have firsthand experience with ECU repair but I know someone that does, and he knows a lot about it. I'll send him a message and refer him here, hopefully he can help you with your question.

 

[blake111]

My cars is doing the exact same thing I think I'm losing fuel bc the pump isn't kicking on with the turn of the key it kicks on when the car cranks then it starts runs for twenty secs then dies like it burned all the fuel out of the line Idk for sure if thats it or not just my guess ny feedback helpful. And I can get 63 psi of fuel but have it set at 43 or so but drops fast after car dies

 

[DroopYeyeS]

Sounds like the ECU side of the MPI relay is not staying connected for some reason blake111. The fuel pump on the 1g's only runs during cranking and running conditions, and does not really have that quick prime that you're looking for at first turn of the key. The car cutting off is probably either your ECU or CAS shutting off.

 

[steve]

If the fuel pump shuts off after you stop running the starter you have damage due to the leaking capacitors. Because of the position of the ECU in the car the leaking electrolyte runs down the board to the connectors and can pool between the pins on the EMI filters under the heatsink where is eats through the traces and soaks into the EMI filter ruining it. Check that ECU pin 56 goes low when the CAS is turning and stays low as long as the engine is turning. Check to see if your getting a fuel pump CEL which happens when the ECU thinks it's telling the fule pump to run but the ECU doesn't see 12v on pin 13.

The fuel pump is only designed to run while the starter is running or while the engine is turning.
Unless you have tuned the car for some other base fuel pressure a 1G automatic ECU expects the base fuel pressure to be 42.7 psi and a manual ECU 36.3 psi.

You questions about the capacitor size and voltage ratings imply you don't know that much about electronics which makes it likely you didn't address all the leakage related problems when you swapped the capacitors. As you noted the capacitors of today are much smaller in size than the 20 year old ones and sometimes that means using a higher voltage rating than original to make the replacement fit. It's also likely that the original work was substandard and even if you did an outstanding job replacing the caps that the real damage was already done.

I usually have to strip down much of the center of the board to fix all the problems and potential problems that the "professional" repair shop just leave behind because it takes too much time.

They'll just swap another quickly repaired ECU for the bad one if it somehow makes it's way back to them rather than spend the time to make sure it doesn't come back.

 

[jmartinez1228]

Under the thermostat housing there are a few sensors. I cant remember exactly which one but one of them broke on my car once and I had the exact same symptoms you had. Check them out and make sure none of the connectors or ends are broken.

 

[steve]

The sensors on the thermostat housing are either engine coolant temp sensors for the gauge and ECU or the thermoswitch for the EGR. The first bunch are electrical, one wire for the gauge sender and two wires for the ECT sensor and the other vacuum lines.

None of these will cause the fuel pump to turn off.

 

[blake111]

Well I have swaped the ecus I have 3 samething so thats not it n srry I have no clue ### or where the mpi relay is can someone explain ###

 

[DroopYeyeS]

As always Steve, you give great pointers. I will double check under the heat sink for damage I may have missed on or near the EMI. I did rub down all areas that I saw leaking gel with alcohol on a que tip before the new caps where installed. If I did miss damage on the EMI, does Digikey sell those as well? As for the sizing, I only asked due to seeing different diameter caps of the same ratings available from different companys even today. I didn't thing use would matter as long as it was electrolytic caps with correct ratings. I just like to be very cautious, . Yeah jmartinez1228, I've already replaced my CTS (2 prong) before this started. Even replaced the wiring as it was age hardend. I've tested the ECU in a friends stock '91 at this point with the same results, so the ECU is definately the problem. I will check the #56 pin this weekend for that drop Steve. I appreciate the info as always.

 

[steve]

I haven't found anybody that sells replacement capacitor arrays for the EMI filters. In the Pre-SMD days they were common but not any more. Before I shutdown the ECU repair business I was going to split a large order with the guys at ECMTuning since the only way to find them now is from a electronics broker, make a replace board to hold discreet SMD caps, or salvage good ones from bad ECUs.

Using alcohol with a Q-tip often redistributes the electrolyte as much as it removes it and lets it cause problems in new places.

Make sure that the traces for the capacitors are clean and bright copper. First because you can't solder to corroded copper, second because the electrolyte often eats the top pads and plated through hole eyelets up disconnecting the top and bottom traces. The new capacitors need to be soldered to both the top and bottom traces. If both sides are clean and the plating intact then the solder will flow all the way through to both sides but otherwise you have to make sure the leads solder to the top as well as the bottom.

 

[DroopYeyeS]

Thanks for the input Steve. I've given up on this board for now. Ordered another ecu for now. I'll retry the board later to see if it can be saved. I appreciate your insight as always sir.