Charles Dehn

OK so Ive been lurking for awhile trying to make sure i didnt bother you guys without properly looking around but I'm completely at a loss and i need help!

so i took a 87 NA motor with 80xxx miles on it, decent compression bout 125ish on each rotor and stuck it in a 88. After a couple months i got it running! It ran great but still had some intermittent starting issues, mostly electrical as the car was converted from a automatic and as this was my first project, there were some growing pains

anyways I start it up to go out and get halfway around the block before the thing cuts out. I was able to limp it home but only with alot of throttle and it sounded like it was starved for fuel. I couldnt get her to start back up again without starter fluid. I had just replaced the fuel tank due to rust the day before so i assumed it was a fuel problem. So my troubleshooting went as follows:

new fuel filter
vacuum leaks (none plus the AFM can be seen being pulled by engine vacuum during cranking)
spark (yes enough for starter fluid)
operation of fuel pump (works. tested with two different pumps)
circuit opening relay (afm triggered)
fuel pressure after filter (35ish)
voltage of injectors at ECU (12.5ish with fluctuations down to 10ish during cranking)
resistance check of AFM (met spec)
12v at injector plugs (12.5ish)
obvious like backwards fuel lines, no fuel in tank etc etc (turns out im not a complete idiot this time hahah)

at this point i assumed the injectors were at fault, perhaps clogged up from bad rusty gas, so i sent them out for testing and cleaning. got them back in (told by the cleaner that they were completely operational) and installed and she still wouldnt start. i then feared for the worst and did a compression check. exact same as before. so i took the UIM back out and tied the injectors to the rail and spun my spare CAS and fuel was shooting out! so i then assumed a air/fuel mix failure of some sort which led me to getting a spare ECU. still no start. I have enough spark and compression to light off starting fluid but not enough or too much fuel. The only things i havent checked so far are the FPR and the water thermo sensor. Could either of those cause this? I took a spare thermo sensor and hooked it up out board since the only parameter its using for starting is ambient temp to adjust the start fuel map. Not quite cold enough yet to matter id think but of course i could be completely wrong so i tried anyways. Still no dice. I dont wanna just throw parts at the car. I suppose the FPR would make sense but I wanted to see if anyone had more input before i spend any more money guessing. could it be an electrical gremlin? anything you guys got is super appreciated i love this car and just wanna see her on the road again

satch

iTrader: (1)

How about unplugging the AFM and jumpering the fuel check connector and see if it starts (if it does don't run it long because it will run insanely rich w/o the AFM). You can check the water thermosensor at the ECU (pin 2I). W/key to on and cold engine it should read 2 to 3 volts. Also, make sure the injectors are the proper resistance as they should be about 11 to 12 ohms if they are high resistance (low resistance needs the resistor box and are close to 2 to 3 ohms).

Xa3r0913

When you put the 87 engine in, did you retain the 88 fuel injectors? 88 models were all high impedance injectors, where 87 was a split year. My early 87 has low impedance injectors with a resistor box to fool the ecu, later 87 and all 88 were true high impedance injectors and do not have the resistor box. There is a 2 pin plug under the hood to jumper to turn fuel pump on even if the afm circuit is open, so there is no point in unplugging the afm , but using a jumper may make a difference.

satch

iTrader: (1)

Unplugging the AFM is done in case the sensor is interfering w/the car's ability to start.

Charles Dehn

so i tried it with the AFM unplugged and fuel check jumpered and it wanted to start! but it just chugged a little bit with the starter motor going. so i deflooded just to make sure and still no start. but it felt like wanted to light off so this seems like the right trail

i swapped the emission harness, injectors, resistor box and ECU all together with the motor. 3 of the injectors read 2.5 and one read 2.4 if i remember correctly from the read out i got back with my injectors after they were tested and cleaned

Xa3r0913

I haven't had an afm interfere with starting, other than one that was stuck closed, so I would've never thought of that. I've also only gone through 3 afms, so I'm sure I'll get there eventually.

Stupid question time! have you checked the resistor box itself? I did have one go out on me a few years back.

Charles Dehn

oh god that could be it. i didnt think to check it since the injectors were getting 12v but electricity doesnt care. ill check and report back. i actually hope im an idiot this time

Charles Dehn

ok so i checked the resistor pack and all four terminals measured ~6v which is what the resistors say theyre rated for on the case.

satch

iTrader: (1)

Volts or ohms?

Charles Dehn

oops i meant ohms hahah yea about 6 ohms

Charles Dehn

also the water thermo sensor checks out at 2.6v backprobed from the ecu

satch

iTrader: (1)

Measure the Brown/White wire at either the TPS or pressure sensor w/key to on and it should read 5 volts.

Charles Dehn

yup im getting 5.1v at the TPS

satch

iTrader: (1)

Did you replace all injectors?

Charles Dehn

yea i had them all tested and cleaned. I even took the UIM and primary rail off and tested them while tied to the rail and fuel was streaming out while i spun my spare CAS

satch

iTrader: (1)

When you placed them back did you by chance put the secondary injector plugs on the primary injector plugs?

satch

iTrader: (1)

You can test the ECU to see if it is trying to fire the primary injectors w/key to start. Pins 3E (Light Green wire) and 3C (Light Green/Black) are the primary pins. Using an LED light you take one lead and place it into the back of the ECU plug for one of the two pins denoted and the other wire lead for the light to pin 3I and w/key to start the light should flicker. Both pins w/just key to on should read 12 volts.

And could you pull the injector plug at one of the secondaries and take an ohm reading as it should read 11 to 12 ohms.

Also, is the AFM connected to the intake duct which runs to the throttle body?

Also, if the LED light does not flicker then measure pin 1G (Green/Black wire) and it should read 1.5 volts in neutral and 12 volts in all other gears w/key to on.

Charles Dehn

im pretty sure i put the primaries back on the right injectors. i dont think the emission harness would fit under the UIM properly otherwise. Also the primary terminals at the ECU measure ~12.5v volts with the secondaries unplugged. I made a little LED light and made sure it worked by hooking it up straight to the battery. while it was hooked up to the injector terminals and the main relay, there was no flashing with the key to start. so i measured the voltage at 1G and got 14-15v in all gears. so this means the ECU thinks the car is in gear when it isnt? the switch itself is at least partially operational as the starter wont crank over if the trans isnt in neutral. also the AFM is hooked up to the intake properly

also satch you are a god among gearheads thank you so much for your help thus far

satch

iTrader: (1)

It says you made sure it worked but it didn't work. Is this correct?

And you might want to ground pin 1G while in neutral and try to start the car. And if by chance it starts just leave it in neutral.

Charles Dehn

yea i was just checking to make sure the light itself was operational. when i backprobed the ECU at the terminals specified, there was no flashing. to ground it should i just run a wire from 1G to the negative battery terminal?

satch

iTrader: (1)

You could use the battery terminal ground. You might want to try the LED flashing test when you ground out pin 1G. And if the light still does not flash then you might have an ECU grounding issue. Pins 2C, 2R, 3A and 3G are grounds and they should read 0 volts w/key to on.

You could also use a mounting bolt for the ECU to ground out pin 1G.

Charles Dehn

i very recently sanded and put dielectric grease on the ECU ground under the UIM as per aaroncakes article when i had the UIM off to access the injectors. i also did all the battery ground at the driver side shock tower, the block at the top of the driver side tower and the starter ground. are there any other grounds that could cause issues?

satch

iTrader: (1)

There is a staple of sorts which is found near the ECU (its job is to group together the grounds at the ECU). If you follow the emission harness as it leaves the ECU and follow it you'll find the staple/clip (should be a within a foot or so of the ECU).

Or you could splice in a new ground which is soldered to the 4 ECU grounds and use one of the ECU mounting bolts to ground the wire to the body of the car.

Charles Dehn

ok so my multimeter is showing all kinds of weird numbers when i checked for voltage at the ground terminals. i suppose adding a new, easy to access but easily hidden ECU ground makes sense. should i run 4 wires directly off the 4 wires that run to the ECU ground terminals or would it be ok to splice all of the ground wires into one, connect them to ground and then splice 4 off of that into the ECU. as far as i know, ground is just ground hahah but if bad grounds are whats causing this, i wanna make sure it doesnt happen again

satch

iTrader: (1)

When testing an ECU wire as it relates to the grounds it is best to use one of the mounting bolts for ground. Of the 4 ground wires some are tied to each other but it is the staple connector which might be the culprit so all 4 need additional wires should be grounded. Cut a slit in each wire about an inch in length. You can pull the sheath off w/your fingertips. Then solder in a wire making sure you don't introduce a cold solder. Then run all 4 wires to one of the mounting bolts (also, obviously tape up or use heat shrinking wrap to protect your work).

Check your meter against the battery ground and see if the meter acts the same or differently so you know what you are up against. Like measure the voltage of the lead coil mounting bracket which should have 0 voltage.