Have a 1986 coupe with about 197,000 miles on it. It has been running fine until this Friday afternoon. It started fine and as I drove through the parking lot, it just died. It would crank, but would not start. Checked for spark on one plug and it was fine. It is posting a code 34 (MAF). I disconnected the MAF connector, but it still won't start. The service manual indicates it might be the ECM. Would appreciate any help. I don't understand why the MAF code is posted without the car running. While cranking the engine does have sufficient oil pressure
(30+).

 

codes will post and remain until you delete. may be old code, unless SES was on.

listen for the fuel pump on for 2 secs when key is turned on. that says FP is operating, may still be fuel issue, filter, regulator, pump itself. eventually you will need to ck fuel pressure, either with a gage or by pushing in the valve to see if fuel squirts out.

verify fuses ok.

if fuel related, may be flooding or lack of. many things besides the ECM. that will be way down the list.

 

Fuel pump starts, checked INJ1 and INJ2 fuses (OK). Pressed on fuel pressure valve and fuel just barely oozes out. Loosed hose at tank and fuel sprays out. Suspect the filter may be plugged.

 

Changed fuel filter and now I get a spray of fuel from pressure valve, but it still doesn't start.

 

So you have spark and fuel pressure. Is there an injector pulse?

 

Can the injector pulse be checked with a digital multimeter or do I need to get a noid light?

 

Noid is best but you may be able to hear them with a length of hose or a mech's stethoscope.

You can, however, use your DMM to check the resistance of each injector by removing its connector and measuring pole to pole. Any that are much lower than the rest are suspect. If you find one or more that are lower than the others then try starting the car with those injectors unplugged. It won't run smooth with only 6 or 7 but it should start and run.

 

meter will work if you put straight pins whatever in the clips to fit each terminal in a harness. I use a 12v test light because I already had it. if you have someone crank the engine it is no problem to hold meter in the terminals.

you are looking for an on/off signal when engine is cranked, not a steady light or reading. normally check one harness per side because there are two circuits for the hot -pink/blk- wires. but one will tell you if ecm is sending a signal.

 

Used a stethoscope on both banks of injectors and they are clicking. Could the timing chain have jumped a tooth? Would't it be back firing if that was the case?

 

not likely. it would still run, yes.

are you sure it isn't flooding from leaking FP regulator- or injector stuck open?

pull an easy spark plug and see if it's wet.

if you ruled out maf, unhook TPS to see the reaction.

 

Pulled a couple of plugs and they looked okay. Unhooked TPS and it still won't start. For some reason, I decided to check the fuel pressure valve again and hardly any fuel came out. Must have some crud in the fuel line and it is plugging up the fuel filter. Will flush the fuel line and install another new fuel filter.

 

wow. not sure if FPR can shut down the flow to that degree. it might be as easy to unhook the incoming line at rail first if you find filter filled.

"easy" and C4 don't go together.

*** don't believe the filter can clog to that degree. would look elsewhere.

 

I would check all plugs, you could have just happened to have checked the plugs on cylinders with good injectors.

I don't know if its the same, but I had a TPI camaro with 1 bad injector that prevented the car from starting. I unplugged each injector one at a time then try starting, which got me up and running on 7 to get home. The ohm test recommended by 94z above will also help ID any bad injectors.

 

Here is the simple, easy way to do several test with one test tool...

Fuel pressure test gauge..

Connect gauge to rails. Key ON. Make note of the pressure if any. Now, with the gauge placed where you can see it easily from inside the cabin, crank the motor. WATCH the gauge needle. IF you had pressure, that is the pretension that is used to fuel start-up. As the injectors cycle with each crankshaft revolution, fuel is USED and the needle on the gauge drops a couple lbs with each inj cycle. The pressure will NOT rise until the engine fires and runs on its own by way of a signal from the distributer.

IF you do NOT see the gauge needle move during cranking, your inj are NOT cycling and that needs to be investigated.

IF you have little or NO fuel pressure, its pump power, pump output, pump screen intank, and/or the inline filter.
IF you CAN hear the 2 second run of the pump at key ON...odds are that its filter.

The FPR will not stop fuel flow. Just the opposite. If damaged the FPR will dump excess fuel into the rails OR in some cases it sends it to the return line back to tank.

#1
Determine IF you have near 40 psi fuel pressure at key ON. Less than 30 and it won;t fire up.

IF pressure is there, inj cycling is the next step in the process.

IF inj are cycling and you have pressure, and spark is KNOWN to be good, then the FPR becomes suspect.

Good luck

FWIW, the fuel pressure test gauge is the mostr valuable tool there is for a C4...or ANY GM EFI motor. Learning and understanding what the gauge and needle behavior mean is the key to quick and easy diagnosis.

BTW...

the fastest way to force it to run LHM or open loop is to unplug the MAF. That forces the ECM to look at the cal-pak for data to run, based on what the TPS is doing. A bad MAF or MAF relay will eventually send it to LHM (open loop) but it must run 8 seconds for the ECM to understand that the data is corrupt and make the shift to OL operation...next to impossible when the MAF is messed up..or the $7 relay.

 

Flushed the fuel line (wasn't dirty) and installed a new fuel filter - still won't start. Got a loaner fuel pressure gauge and the most pressure I ever saw was 15 psi. Replaced the fuel pump, strainer and bypassed the pulsator with injector hose. Still low or no pressure and still won't start. Could the regulator be ruptured and leaking off the pressure?

 

how long did pressure hold? may be returning to tank or leaking from injector or FPR.

pull the vac line from the FPR if you have not yet checked that. with pump on, there should be fuel spraying from the FPR if the diaghragm is damaged. that fuel will spray into the plenum, flooding the engine. but should flood all plugs, not a few.

if ok, leave vac hose off and crank to see if pressure builds. if you are sure no fuel is leaking past injectors, it has to be returning past the FPR

 

There is a procedure that involves clamping the fuel supply and return lines at the gas tank that will help identify what the cause of your pressure drop is. I'm pretty sure a search will turn it up, if its not also in the FSM.

 

I took another look at the plugs and they are wet, so suspect the regulator is bad. Thanks to everyone for all the help.

 

[QUOTE=joe paco;1581969853] pull the vac line from the FPR if you have not yet checked that. with pump on, there should be fuel spraying from the FPR if the diaghragm is damaged. that fuel will spray into the plenum, flooding the engine. but should flood all plugs, not a few.

if fuel is not leaking from the vac hose connection it is not a damaged fpr. that leaves one or more injectors leaking. if you verifed that all are clicking, that means the ecm is controlliing them, but an injector can still leak when closed. it may ohm at 16, still leak because the "needle" does not seat.

joe

 

Who ever said to check the fuel pressure (guage attached) while cranking the engine was wrong! Attach the guage to the fuel rail..then turn the key to the on position (do NOT crank) the fuel pump should run for 2-3 seconds, pressurizing the rail...(to lazy to go get my FSM's) once you do...watch to see if the pressure holds or drops rapidly (if it drops quickly) you have a leaky injector(s).

Could also be the connectors to the fuel pump.

You mentioned oil pressure. Our early C4s have a fuel pump relay, if that fails the oil pressure switch activates, and only 4 lbs. pressure is required for the fuel pump to activate.

Checking the injectors with an ohm meter only tells you the health of the coils in the injectors.....not wether it's leaking or not.