So I finally got the truck to run pretty good but is hard to start.
I get 2 sec fuel pump at key on, no pump during crank then pump on after crank stop. I have crank signal at c9 all the way into ECM. If I jump fuel pump prime it will start then run with prime lead dropped. I have tried a different chip and different new ECM. still does it. What else can cause this I am getting pissed off with it?
I don't think the crank signal has anything to do with turning the fuel pump on or off. Outside of inital fuel pump prime, two things turn the pump on.Originally Posted by mjc
-the signal from the pickup coil in the distributor
-the oil pressure switch
I'm assuming you are getting no signal from the pickup coil. The only thing that keeps the fuel pump on while cranking is the signal from the pick-up coil. After the oil pressure builds up enough to close the switch, it can run off of the oil pressure switch. Are you using an 8-pin ignition module?
1986 ski centurion boat, 351W, gt40P heads, edelbrock performer rpm, 1227747 ecm, 72 lb/hr injectors (@18 PSI fuel pressure), .490/.490 lift, intake 278 duration, exhuast 282 duration, 112 lobe separation
1969 camaro, 355ci, performer RPM intake, camel hump heads, 1227747 ecm, 454 throttle body (@18 PSI fuel pressure),
2016 Chevy Cruz
2014 Chevy Silverado
IU think it is only a 4 pin module. what I have read is pump should come on during crank. it is a 7747 ECM. motor is older so it can take a little bit for oil pressure to come up to turn on pump during crank. I am wondering if I just got a bad ECM?
Dark Green/White from A1 turns on pump and Tan/White B2 finds signal. Both wires need to be hooked up. Sounds like you have A1 and not B2?
1990 Chevy Suburban 5.7L Auto ECM 1227747 $42!
1998 Chevy Silverado 5.7L Vortec 0411 Swap to RoadRunner!
-= =-
I will look for that tomorrow
Finally got back to this. All wires are hooked up and come on when they are supposed to(i think) but a1 drops to 9.3v when cranking which isn't enough to pick relay. I tracked it back to the pink wire coming out of bulkhead connector. Now just need to figure if there is to much load or some other cause.
Weak battery? Have it fully charged then load test. Any parts store will do this free if you don't have equiptment. Could be a weak or dead cell in batt.
1990 Chevy Suburban 5.7L Auto ECM 1227747 $42!
1998 Chevy Silverado 5.7L Vortec 0411 Swap to RoadRunner!
-= =-
Battery is new. It look like the + cable has to much voltage drop during crank. I jumpered from pos terminal to the firewall terminal block and it fired right up so going to replace that cable and check connections at the starter.
Cool!
But Batt is new means nothing nowadays with parts.
1990 Chevy Suburban 5.7L Auto ECM 1227747 $42!
1998 Chevy Silverado 5.7L Vortec 0411 Swap to RoadRunner!
-= =-
Battery checks good at 2 different stores. I can get it to start whenever I need to by just cranking it a few times until oil pressure builds up. It seems to run good except when accelerating I can get a small back fire and acceleration is slow. Runs good at other times. Checked the cat and per GM manual it is good(still may remove it). I was checking for timing advance and I see very very little if any. I would like to try my HEI dist. Will it work by just dropping it in and wiring it up or is the lack of the computer controlled dist going to make it not run?
Forget the HEI over the Small cap EFI distributor. Why go backwards in tech?
Timing advance won't show much on timing light because there is no load on engine increasing MAP kpa = decreasing vacuum as shown in spark advance table.
The starting OK when oil pressure is up and not when oil pressure below sounds like faulty Fuel Pump Relay as it is a 2 part system and oil pressure will close the circuit and send power to fuel pump.
Back fire upon acceleration could be low fuel pressure or a tuning issue needing more AE, Acceleration Enrichment.
1990 Chevy Suburban 5.7L Auto ECM 1227747 $42!
1998 Chevy Silverado 5.7L Vortec 0411 Swap to RoadRunner!
-= =-
How do I check the AE? I have good fuel pressure right at 13. I still have a low voltage during crank at fuel pump relay that keeps i think It keeps from picking druing crank.
Have you checked the fuel filter? Could be clogged, not allowing enough fuel during initial prime?
fuel tank/pump/filter are all new if I jumper pump it starts ok. I just have crappy accleration that really bothers me.
Any modifications to engine from stock will need tune work. Data logging for AE is very tricky but can be done with monitors in TunerPro by watching the INT when accelerating, Wide Band is easier to dial it in perfectly but not necessary. Get a data log of the issue.
What engine, vehicle, mods etc...?
Could also be a mechanical issue like MAP sensor not plummed to rear port of TBI between fuel lines, vacuum leak, sounds like you have great fuel pressure, EGR issue...injectors that need service...
But you seem to be going low voltage at crank? Batt can be brand new and still have issues. Or biggest reason would be a Batt with not enough cranking amps for EFI, which needs much more then a points system type engine. Can log on some systems during crank, or check power at ECM with DVOM and see how low voltage is during cranking. You could check right at the Batt as well but this would not tell if there's even more voltage drop to ECM. IIRC when they drop below 10 volts during crank issues can arise. 9 volt is really pushing it, 8 volt and forget it! You need MORE POWER!
1990 Chevy Suburban 5.7L Auto ECM 1227747 $42!
1998 Chevy Silverado 5.7L Vortec 0411 Swap to RoadRunner!
-= =-
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