Cranks but wont fire....Plz help ASAP

[ReddMan]

I would like to apologize in advance if this post seems excessively long, I just wanted to make sure to cover everything to help get an accurate diagnosis.

So I have a 94 Mazda Navajo. Its the exact same thing as a 94 Ford Explorer Sport. The problem that I'm having is that it wont start. This is my girlfriends daily driver and with her baby around we need to get this thing running again pronto.

What happened:
One morning on the way to the store we started the truck and it ran for about 30 seconds and died. It proceeded to sit there and crank and crank and crank for about ten minutes before it decided to start again. We got to the store, went shopping, came out, and it refused to start. Crank crank crank. We had to tow it home and it hasn't started since. It would start and run for about five seconds when we sprayed ether in the intake but that's about it.

No work was done to it around this time. I changed the spark plugs about 2 weeks before but there were no problems.

What I've done so far:
New fuel filter
New fuel pump
New plug wires
New spark plugs (for the second time in less than a month, the first set weren't pre-gapped like they were supposed to be. Got the nice ones this time....)
New ground wire (the old one was corroded for like 8in....)

After all this it still would just crank. We called a backyard mechanic that we met and had him come take a look at it. He used a voltmeter and we learned that the injectors are getting 12V but not pulsing. Also our fuel pressure didn't seem to be where it should, but we don't have a fuel pressure gauge so I can't be sure. We also tested the coils to make sure that they are all good and there wasn't any problem there. Also tried to reset the Inertia Switch, but where the manual says there is a button no button exists. But the new fuel pump kicks on (when it wants to....:thumbdwn so I don't think thats the problem. Also checked the Fuel Pump Relay, no problem there.

I looked on here for what could cause the injectors to not pulse and came back with either a problem with the ECM, the Crankshaft Sensor, or our mechanics idea of the Fuel Pressure Regulator.

Today me and her uncle went to the junk yard (new ECM $200, junkyard ECM $15) and got all three of those parts. It was almost dark when we got back and since I need this truck running ASAP I put all 3 parts in instead of doing them one at a time. I know that was probably a bad idea, but its already done.

Now with the new (to this truck) ECM, Crankshaft Sensor, and Fuel Pressure Regulator installed there is absolutely no fuel pressure, and of course it still wont start. I'm at a complete loss here and I have no idea where to go next. We are running low on funds for this truck so basically and other part that goes on it at this point is coming from the junkyard.

What do you think could be causing this? Do you think its one of the parts I just replaced and they are just bad or could it be something else that I don't know about? Any and all input would be greatly appreciated.

Thank you

 

[MrShorty]

It would start and run for about five seconds when we sprayed ether in the intake

This usually points to a problem with the fuel system.

Also our fuel pressure didn't seem to be where it should, but we don't have a fuel pressure gauge so I can't be sure.

IMO, a fuel pressure gauge is a necessary tool for someone who is serious about doing their own diagnosis on a fuel injected engine. If you don't have a fuel pressure gauge, borrow one (I think autozone has a gauge set among their loaner tools) and measure the fuel pressure.

But the new fuel pump kicks on (when it wants to.

What does this mean? That the pump is not running consistently? The pump should run for a few seconds after turning the key on, and run continuously when the engine is rotating. If it is cutting out when it should be running, that sounds like a good path to pursue.

 

[ReddMan]

What does this mean? That the pump is not running consistently?

No. Like I know that the fuel pump should turn on every time I turn the key on, but it doesn't.

This is what lead us to believe it was the fuel pump in the first place, but when we removed the old one and hooked it up to the battery it ran. It looked pretty rough so we replaced it anyways. Then we thought it might be the inertia switch, but like I said there is no reset button to be found on it. But the fuel pump will occasionally run which leads me to believe its not the inertia switch (right?) which led me to the relay. I know the relay isn't bad because I switched it with another relay with the same number and everything still worked. So between the fuel pump acting up like that and the injectors not pulsing I figured it might be the computer. Now I'm left believing that either the computer I got at the junkyard was bad (both of them looked completely fine but i dunno....) or there is a problem with the wiring to the fuel pump. If its a wiring problem I have absolutely no idea what to do....

 

[wantdeath]

my fuel pump wont run when there is still enough pressure in the line. like if i cycle the key a few times, it wont turn on the pump unless i crank it again. then it will do it all over again. mine runs fine. i read somewhere in this forum about the ecm must be the exact same model number or it might not or wont work right or at all. maybe verify the part numbers on the ecm. you could have gotten a bad ecu at the junk yard.

 

[MrShorty]

I know the relay isn't bad because I switched it with another relay with the same number and everything still worked.

The problem with this strategy is that, if you swapped the fuel pump relay with the EEC relay, you may not see much difference in what the fuel pump is doing. Because the PCM is responsible for turning on the fuel pump relay, both of these relays have to be good for the pump to run correctly. It looks to me like we still need to verify that the relays are working.

If its a wiring problem I have absolutely no idea what to do....

And that kind of looks like the direction we are headed with this. Cars are becoming more electrically complex, not less, and with a '94, you are already 18 years behind the curve. If you are serious about doing your own diagnosis on your car, it is important to have at least a basic understanding of DC electricity/circuits so you can diagnose a simple DC circuit like the fuel pump circuit. If you are willing to deal with the tedium and frustration of it, the fuel pump circuit is probably a good one to learn on. Unfortunately, I'm a terrible teacher over the internet like this, which will only add to your frustration (my apologies in advance).

1st thing to do is get a wiring diagram. Even the one in Chiltons is good enough, and, if the print version is too passe for you, then autozone has a free version of chiltons available on their website (if you are willing to register with them). Some public libraries have subscriptions to online manual services, so that might also be a resource for this information electronically. To test the circuit systematically, you start at the battery, and work your way down the circuit, looking for a significant drop in voltage. Where the voltage drop is is where the problem is. The relay can also be a good place to start, especially if you use the "test lead" in the self-test connector.

 

[ReddMan]

@wantdeath- yeah i read that too i got the exact same one we took the old one in with us to make sure they matched. I also heard somewhere that if the ECM/PCM is bad you should be able to see a burn or blemish in the board. Both the old one and the replacement look to be in great condition. One bad thing if the new one is bad is that I gave them the old one for the core charge....

@MrShorty- Thanks for the info on the relays, I didn't know that. I'll have to look into that a little better. Also I'm about to go register with autozone right now in hopes of a free online Chilton's lol. I know its horrible but I actually don't even have the book on this truck. So far I've gotten everything I need from right here.... As for learning the electrical, your right it is important to know and this will probably be a good way to learn. That really doesn't make it suck any less....

Thanks for all the info so far.

 

[1991explorerEB]

did you do the fuel pump on the tank. and make sure that there is power going to that pump and from it.

 

[ReddMan]

did you do the fuel pump on the tank.

Yeah I changed the fuel pump in the tank. Also replaced both the fuel pump relay and the eec relay today. The fuel pump runs like it should now but the injectors still aren't pulsing. The fuel pressure regulator I got was bad so we put the old one back in but I'm not sure if the new crankshaft sensor we got is bad too or what the problem is. I might just have to go buy a brand new one....

 

[MrShorty]

The fuel pump runs like it should now but the injectors still aren't pulsing.

If you look at a wiring diagram, you see that the injectors should have a constant 12 V power supply, and are ground side switched. Your '94 should be SEFI, where each injector has it's own "switch" in the PCM, and it is not likely that all of them would fail at the same time.. So, when all 6 injectors are dead, I would start by checking that the injectors have 12 V.

 

[ReddMan]

when all 6 injectors are dead, I would start by checking that the injectors have 12 V.

Yeah we checked them with the voltmeter and they are getting 12V, but from what I was told they are supposed to pulse and thats when they send fuel to the cylinders. I was also told that I need a diode tester to check them, but I don't have one.

It is my understanding that the crankshaft sensor is what sends the signal to the injectors to make them pulse, which is why we replaced it. I'm wondering now if the one we got is bad or if there is something else that could potentially cause this problem?

 

[MrShorty]

It is my understanding that the crankshaft sensor is what sends the signal to the injectors to make them pulse, which is why we replaced it. I'm wondering now if the one we got is bad or if there is something else that could potentially cause this problem?

A bit oversimplified. The CKP is a basic Hall effect sensor that tells the ICM when TDC is, and how fast the crankshaft is turning. The ICM passes that information on to the PCM. The PCM then takes that information and all the other information it is getting to decide when and for how long to fire each injector. So the CKP doesn't directly tell the injectors to fire, but the CKP is an important input in the PCM's decision to fire the injectors.

Have you pulled codes from the computer? A lot of faults on this side of the decision to fire the injectors will leave clues in the computer's memory.

Have you checked the computer's power and ground circuits?

 

[ReddMan]

Have you pulled codes from the computer? A lot of faults on this side of the decision to fire the injectors will leave clues in the computer's memory.

Have you checked the computer's power and ground circuits?

No, I haven't pulled the codes yet. I don't have hardly anything for tools, including a light tester (the voltmeter was a friends). We are supposed to have a guy coming out this weekend to run a diagnostic on it.

As for the computer like I said we replaced the ground wire because it was super corroded, but no I haven't checked the circuits. Again with the lack of the light tester, plus I'm not exactly sure how to go about that.