actualsize New Reader
March 16, 2010 11:47 p.m.

I'm in the midst of rebuilding my 20-year old Miata, and I have a tight deadline before the show at Laguna Seca in April. But I've hit a snag that falls outside my area of expertise.

The motor (a confirmed runner) is in. I've reinstalled all of the harnesses and connected everything back up. The motor turns over, and it stumbles (only once per start attempt), but it will not catch and start.

I cannot hear the fuel pump. I don't hear the injectors pressurizing.

The fuel injector relay by the washer bottle is engaging (I can hear it). I can measure 12V at the pump connector, but only in the "crank" position. There is no voltage back there in the "run" position. I do not hear the under-dash circuit opening relay clicking over in the "run" position.

That changes when I jumper F/P to GND in the diagnostic connector. When I do this, I hear the Circuit Opening Relay under the dash click in the "run" position, and I get 12V in both "run" and "start" at the pump. But I still can't start the engine and I still don't hear the fuel injection system pressurize.

What's more, when I key off after a couple of cranks, I hear a very loud clunk in the MAF box that I've never heard before. I presume something is banging shut after the voltage drops away, but this sound is new to me.

The engine, and mass airflow box came straight out of a running car, so they are not suspect. The fuel pump may or may not be good, but I need to make sure it gets 12V in the "run" position before I write it off. I still don't have that without a jumper in the diagnostic connector.

I have the wiring diagram, but I'm simply not smart enough when it comes to electrics. This car was torn down to a bare tub, but when I put it back together I was careful to make sure everything went back that way it had been before. I am confident that all the under-dash grounds have been reconnected, but not certain. I don't see anything astray under there.

Does anything jump out at any of you? I'm really stuck right now. Ask me anything. I'm not sure where to start to diagnose this.

Keith SuperDork
March 17, 2010 12:27 a.m.

Are you sure you have the fuel lines hooked up correctly? Back of the rail goes to the front line, front of the rail goes to the rear line. It's the most popular whoopsie on an engine swap.

You're not going to get 12v in the "run" position unless the car's actually running - there needs to be air flowing through the air flow meter. And the "bang" is probably the big flapper door in the air flow meter shutting after the air stops flowing. I've never heard that happen, but I don't work on cars with the stock AFM very often these days.

actualsize New Reader
March 17, 2010 8:59 a.m.

I'll recheck the fuel lines, but they're only long enough to fit one way. It doesn't look possible for them to be reversed from their current position. I'm used to hearing the system make some kind of high-pitched noise for a second (then stop) as I turn the key to on, presumably as the fuel system pressurizes in anticipation of a start. But I'm hearing none of that.

The AFM worked the last time the engine ran two weeks ago. I swapped them both into this car as a unit. No reason to think it crapped out while sitting in my garage. I will test it, though. I had not thought of the thing with the door and the assistant. That's an easy test. More ideas like this, please!

The fuel pump has sat dormant in this car for 9 years. I'd more likely believe it's merely seized or gummed. How can I test the operation of the pump motor without first removing it from the tank? I can easily swap it out for a known good one, but that's something I don't want to do until I check this one first.

But after a few cranks, I do smell a bit of fuel, so it may be working after all. It could be spark, I suppose. I do get a tentative stumble as at least one cylinder fires, so there's at least some spark and some fuel. Wires seem to be connected to the correct plugs. They're good wires and plugs, from the same engine. Only the coil pack is different. (The restored car is a very early production model. The male and female connector ends of the wire harness and coil pack are reversed with respect to every other Miata I've ever seen. So I had to use the original coil pack, which I'd kept stored in my garage.)

BoxheadTim HalfDork
March 17, 2010 9:05 a.m.

The high pitched noise should be the fuel pump pressurising the system so I'd be having a closer look at that.

Sofa King Reader
March 17, 2010 9:15 a.m.

Why don't you try running it with starter fluid. If it runs for a few seconds you can rule out everything other than the fuel system.

actualsize New Reader
March 17, 2010 9:17 a.m.

In reply to BoxheadTim: Exactly, but I'm not hearing that at all. That's kind of why I'm concerned.

Keith SuperDork
March 17, 2010 11:04 a.m.

Rule is - always go back to what you just did. It's highly unlikely the AFM just died coincidentally with an engine swap.

The best way to check the fuel pump would be with a fuel pressure gauge. Throw on that FP jumper in the diagnostic box. Quick and easy. For quick and slightly more hazardous, disconnect the feed line from the rail and stick it into a bottle, then turn on the ignition to see how much flow you're seeing. Remember, the feed line goes to the end of the rail without the regulator, the regulator is on the return.

With the jumper in place, you don't need airflow through the AFM to keep the pump running. Even without the AFM working and the jumper, the car should start and run for a couple of seconds as Wonko described.

I'm still voting for reversed fuel lines. It's definitely possible to do. It's been done a bunch of times. Very popular problem.

Keith SuperDork
March 17, 2010 1:20 p.m.

The only way to screw up the plugs is to hook them to the wrong coil. There are two posts on each coil, but it doesn't matter which one you use as long as the correct pair of plugs are connected to the correct coil. 1 and 4 should be hooked to the driver's side coil on a 1.6, if memory serves.

actualsize New Reader
March 17, 2010 11:08 p.m.

The MFA flapper test passed with flying colors. I have power when I should and all of the relays work as they should. So at least the wiring harness went back together right.

The fuel lines on the engine end look OK, but I just remembered I took the tank and fuel filter out of this beast, so the lines could be crossed in either of those places. If that stuff checks out I'm going to assume the pump is toast, or at least clogged with varnish.

As for the coils, the plug wire number they correspond to is molded right onto the plastic near each receptacle, as far as I can tell.

actualsize New Reader
March 18, 2010 12:20 a.m.

Plug wires OK, fuel hoses correct. But when I removed the fuel return line at the engine, it was dry; no fuel dampness at all.

Fuel gauge on the dash also motionless despite 5 gallons of fuel in the tank. A sequence of five beeps from the dash repeats whenever the key is turned to "run" with the "Engine" and "Charge" lights on steady.

Confirmed good voltage at the end of the harness at the top of the tank: 12V @ "run" with MAF flap open any amount, 12V @ "start".

The dry return line and the continued lack of fuel system pressurization noise is making me think I have a dead pump, but I do not have a fuel pressure gauge to confirm it. I can't explain the occasional fuel smell or the dash beeps/lights, though.

Tell me why I'm wrong!

pigeon HalfDork
March 18, 2010 1:38 a.m.

Can't help on the fuel problem but the 5 beeps repeated 5 times is the airbag computer telling you that the warning bulb in the dash is blown or missing, and that there is a fault in the airbg system.

BoxheadTim HalfDork
March 18, 2010 5:04 a.m.

actualsize, do you have a spare pump you could drop in?

That said, I would think that the lack of fuel gauge movement might indicate that while you've got power to the connector, there could be a broken connection further on that prevents both a fuel reading and the pump from getting power.

I assume you're positive you've got fuel in there?

Keith SuperDork
March 18, 2010 7:52 a.m.

You may not have a fuel pressure gauge (they're cheap and useful), but you check for pump operation by simply unhooking the feed line to the rail and putting it in a jar.

The power for the pump and the gauge signal come from the same connector on top of the tank. You said the tank and pump were out of the car - is there any chance this didn't get plugged in?

actualsize New Reader
March 18, 2010 10:12 a.m.

@Keith and @BoxheadTim: I have the tank inspection panel off and I'm taking my voltage readings in the various key states and MAF flap positions at that very connector. So I know 100% for sure that the connector is plugged in and the correct voltage is being delivered to the pump via the connector at the top of the tank when I attempt to start the car. I will swap in a known good pump tonight. I can do a dual exchange with another car. I didn't want to do that until I was pretty sure that was the problem. I think I'm there now. I'll try taking the inlet hose off at the fuel rail and doing the jar trick first, but the dry exit hose makes this step seem unnecessary.

@pigeon: Damn! I thought I was solving my problem once and for all by removing the airbag warning lamp bulb while I had the IP out of the car. Guess I have to put it back in and reinstall the electrical tape over the flashing icon. The trouble with the airbag system is that it is entirely missing from the car, on purpose! No biggie -- tape will be authentic for the restoration.

davidjs Reader
March 18, 2010 10:29 a.m.
actualsize wrote: @pigeon: Damn! I thought I was solving my problem once and for all by removing the airbag warning lamp bulb while I had the IP out of the car. Guess I have to put it back in and reinstall the electrical tape over the flashing icon. The trouble with the airbag system is that it is entirely missing from the car, on purpose! No biggie -- tape will be authentic for the restoration.

I believe a 3 (30? 300? it had a "3" in it) ohm resistor will make the beeping go away... If you look around for Miata's with replacement steering wheels, I'm sure you'll find the number...

MCarp22 Reader
March 18, 2010 10:35 a.m.
actualsize wrote: @pigeon: Damn! I thought I was solving my problem once and for all by removing the airbag warning lamp bulb while I had the IP out of the car. Guess I have to put it back in and reinstall the electrical tape over the flashing icon. The trouble with the airbag system is that it is entirely missing from the car, on purpose! No biggie -- tape will be authentic for the restoration.

Just unplug the blue airbag module under the dashboard. Problem solved.

actualsize New Reader
March 18, 2010 1:40 p.m.

In reply to MCarp22: I knew that once, then forgot. Thanks for the reminder.

Keith SuperDork
March 18, 2010 2:47 p.m.

If you're checking voltage right at the pump, then I'm voting for a dead pump due to years of old fuel. Probably smells like my MG

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