I replace my fuel filter few days ago since then the car wont start, any thoughts if this could be the cause ?
Well there are many unrelated issues that could cause that, but given the area you were wrenching, take a look at the wires for the external fuel pump? Does it audibly run for a couple of seconds with every key-on? (open the trunk and listen).
If itās not that, then we can move on to other things.
~Paul K.
Without more details, Iād say the filter might be in backwards or the fuel inertial cutoff might be stuck closed.
When key is on i can hear the fuel pump for few second
If you take the air filters out & open the throttle butterflies, do you smell gas in the intakes?
If so, check your ignition. Do you have spark from the coil to the top of the distributer cap, and from there, to any plugs?
(Weāll get to amp problems and signal problems to the computer if needed. But lets start here.)
~PaulK
So easy to put the filter in backwards, a cave man can do it.
Yeah, but would a filter installed backwards prevent the engine running?
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Iām with you Kirbert. I donāt think fuel filters have a check valve usually. Installed backwards Iād think it might restrict flow, and put more wear on the pumpā¦ but I wouldnāt first think it would stop enough flow to cause a starting problem. Then again, itās a dead easy thing to check and correct.
~Paul K.
No, The engine will still run relatively well even with the filter backwards. (I know from experience)ā¦
Before I replaced the fuel filter, the ignition worked great, so my direction was to check something related to the filter
Iād think youād want to check electrical stuff in the trunk, like perhaps you bumped something. If that doesnāt pan out, weāre probably going to have to investigate coincidence: Something wholly unrelated just happened to fail as you were fiddling with the filter.
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Yes, like while I was fiddling with the fuel pump relay I accidentally made a short circuit. After that the car refused to start no matter what.
After days of swearing and frustration I discovered that a piece of debris had entered the pump and the pump was jammed.
I checked the filter flow direction and its o.k
I also check the fuel pump to see if there fuel flow from the tank to the filter and its also o.k.
I think my next step is to do a fuel pressure testā¦
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Checking fuel pressure is a decent idea, but the only way I could see there being an issue (since your fuel pump runs) is if somehow the pipe got a clog in it, or the front regulators are not holding any pressure (coincidence). Really, all the other explanations of it not starting are coincidence.
How this stuff works when everything is working:
You turn the key on, The main relay is turned on, which provides power to the ECU and to the fuel injectors. The key switch provides power to the +side of the coil and to the + side of the Lucas AB-14 black-box ignition amp (really just an enhanced GM 4 pin HEI amp in the box).
The ECU wakes up, and turns on the fuel pump relay (by grounding it) which allows direct battery power to pass through the FPRelayā¦ through the pump spinning it, and back to ground. The FPRelay gets itās contol-side power from the main relay.
When you turn the key from ON to START, +12V goes to the starter relay triggering the starter motor. Simultaneously the +12V signal is wired to the ECU so that the ECU knows the starter is turning. The ECU then re-starts the fuel-pump. It may or may not provide some starting injection pulses. There is debate on this and it is know that the later 26CU ECU does not do this, and waits on the RPM signal.
Assuming the ignition is working, the VR sensor in the distributer begins generating an AC waveform everytime a tooth of the 12point star wheel passes the pickup sending that to 2 pins of the amplifier (2 pins of the GM amp). The other two pins are the +12V shared with the coil, and the negative side that causes coil discharge by going open circuit. That negative side is split 3 ways (wiring) inside the amp. Off the negative pin is a direct connection to the neg side of the coil, and a path to a white silicone blob containing resistors. Two wires spring from that. One to the Tacho counting RPM pulses, and one critical one through a shielded white wire to pin 18 of the EFI ECU. Without that RPM pulse the ECU will at least not continue injection, if even starting injection at all. This wire is often brittle and breaks causing no RPM signal to the ECU, thus no injection. (A dead amp can cause this too, as well as problems with the internal enhancements like the suppression capacitor).
But if it does, work, then the ECU calculates the required fueling based on RPM and a reading from the Temp Sensor and begins injection by GROUNDING the injectors bank-to-bank first directly, and then switching pathways to through the resistor pack to HOLD the injectors open while limiting current.
If the temp sensor has a problem (bad wiring, corrosion, open circuit etc) the fueling usually goes wild overfueling to infinity, also causing non-start but flooding.
A bad connection at the resistor pack can cause problemsā¦ usually not non-start, but itās often a good connection to clean and ensure good contact.
Other issue that can arise: Bad wiring in the injector loom due to age/brittleness. A failing ECU (not often but possible).
The other sensors provide trim and just arenāt as critical for ā¦ at leastā¦ starting. That includes the IAT (air temp sensor), TPS (throttle position sensor), O2 sensors (they donāt come in play except after theyāve heated up and at cruise), and even the MAP sensor signal (the car will run badly without it at some RPMs).
Another thing to look/listen for is with key/on cycle the throttle turnstyle to WOT (wide open throttle). IF the WOT switch is working, you SHOULD hear the injectors all burst at once when you do this indicating that the ECU is capable of commanding them.
~Paul K.
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Paul, thank you for you replay,
Iāll check the fuel pressure first to see if there is any pressure issueās, then iāll look for any problems with the fuel regulator (!?!?!).
I have another question, when i crank the engine i should hear the fuel pump runing ?
Yes.
You have to have someone back there to listen though, itās a very faint noise and you canāt hear it in the cabin when the starter is running.
Better idea is connect a test lamp to the pump somewhere you can see it.
O.k
So i crank the engine but the fuel pump doesnt work, where i should check for the problem ?
To quote myselfā¦
The 6CU or 16CU (you have one or the other) both receive a +12V signal event from the Start Relay on the Right front fender at the rear. That +12V signal is split to go to the starter, and to the ECU pin 26. The belief is that itās THAT signal that signals the ECU to re-ground the fuel pump relay allowing the fuel pump to work, and beyond that, when the car has started and is running, the ECU continues to keep that fuel pump relay grounded (keeping it activated) to keep the fuel pump running during engine running. (We donāt actually haveā¦ or I canāt findā¦ a definitive answer on this particular part of the sequence from the expert Roger Bywater, but the thought is the start signal from the start relay isnāt there for nothing, and re-starting the fuel pump makes sense.) As noted, the later 26CU (1992) does not have that start relay input, and must rely on the RPM input for startup fueling, and restarting the fuel pump.
The 6CU is known to be an item that can fail to ground the fuel pump relayā¦ BUT~! AFAIK as long as you hear that fuel pump run with key-on, there IS no failure in the ECU.
Checking pressure in fuel rail is a decent idea, although I really lean towards signal problems here. One way to ASSURE pressure is built in the fuel rail is to use hose clamps on the outgoing hose past the fuel regulator on the outgoing side (the important one).
I should also note, that as a temp fix for those that have had 6CU fuel-pump relay control failures, or for testing, the trick is to remove the fuel pump relay, and put a jumper wire across pin 30 and pin 87 forcing the fuel pump to run under direct battery power (jumper must be manually removed every time the car is shut off).
~Paul K.
So today I tried that the fuel pump will run continuously by putting a jumper from pin 75a to 75 and to pin 30, no luck the, the car still not startā¦