Mazda Experts? Our Mechanic is Stumped...

[Laneco]

Our daughter has a 1993 Mazda MX6 LS with an auto trans and the 2.5 V6. We've had a couple of these and they're pretty reliable little cars. This one is the lowest miles one we've had at 160K.

When I bought it for her, at 150K, I had the timing belt done, plugs, wires, etc. Very regular maintenance. Abruptly, last July, the car cut out and died. It has been at the mechanic's ever since and our daughter has been using Helga, my Volvo wagon.

The mechanic is a lifelong friend of my husband and is a GOOD mechanic. But he says he's flat stumped on this. The car has full spark, compression, and good fuel pressure. But the injectors will not fire. He has swapped in two seperate known good computers. Still the injectors will not fire...

The engine will run quite happily while manually spraying into the intake. Ignition system is absolutely fine. But the injectors will not fire. The car does have a factory alarm system.

Any ideas on what I can suggest to him?

angela

[9dreizig]

Sounds like it might be timing related ? Meaning if the computer isn't getting the signal it won't fire the injectors.. Is it ODBI or ODBII ? is it throwing any codes ? Might be time to take it to the dealer ??

[Porsche-O-Phile]

Start simple/obvious. Do the injectors actually flow? Has he pulled one or more out and tested to see if he can get them to open with voltage across the terminals? That's step #1. Next is fuel pressure - is there pressure at the rail? Pump-related?

Someone mentioned timing - are the injectors firing at ALL or just totally zero flow all the time regardless of engine position?

No leaks or fuel supply lines popped off anyplace (if it was between the tank & pump you might not ever know except the pump would be whirring and sucking air...)

[Jared at Pelican Parts]

Check wiring continuity and flow to the fuel injectors.

[krystar]

crank position sensor.

[kaisen]

Crankshaft position sensor

[doug_porsche]

Originally Posted by krystar

crank position sensor.

Probably not. Without that working you would not have spark.

Bad ground on the Fuel Injector side?

[Paul_Heery]

Angela,

You mentioned in your original post that the car has a factory alarm. I picked up my 951 for a song because it had similar problems to your daughter's car. Well, after chasing down through the wiring schematics, the factory alarm became my main suspect. After disabling the alarm, the car started right up. Your problem may be similar.

I looked on the MX6 forum and found that disabling the alarm is an relatively easy task. You might want to give this a shot and see if that addresses the problem.
Disable factory alarm--easy way! - Mazda MX6 Forums: MX6 Forum

[hardflex]

Injector ground maybe? if they're all grounded the same place.

In troubleshooting toyotas there was a circuit wire that transmitted a signal back to the brainbox that the spark had indeed occurred. If it did not receive that signal the brain would shut down injectors in order to prevent damage to the Catalytic Convertor. So if that wire was broken you'ld get a pop at most before it shut down. Maybe some variant of that on the Mazda system

[Laneco]

Car is OBD-1 (in other words, pre-OBDII).

Fuel pressure is measured at the rail - it is good.

None of the injectors fire - zero.

Really leaning toward the crank sensor. After seeing your posts, I did a search and found that the crank sensor sends a signal that phases the firing of the injectors. That search yielded a few test sequences to narrow the issue.

I would have thought a failure of that sensor would have also resulted in a failure of the ignition system (no trigger). Perhaps not... The part is $80 and at this point, I'm ready to just buy the part and have it put in. It works or it doesn't. Six months of buying insurance for a car that doesn't run and being without my own car is really starting to piss me off.

I'm going to bypass the alarm. It's had a glitch in it for a while. Disabling will take care of that.

angela

[Scott R]

There was an issue on these motors with the air vane sensor dying. The vane air flow switch/sensor has to see input air in order to run the fuel pump during cranking and running.

I can't remember much else, but remember seeing it on multiple MX6's when I was wrenching. It was an easy repair.

[yetibone]

Check for B+ at the white/red wire on any injector. The main relay is working if the ignition coil is firing, but there may be a break, or short in the wire feeding power to the injectors.

[9dreizig]

Hey Angela, I"m pretty sure Sid (porsche4life) had the exact same issue with his 944.. same symptoms

[nocarrier]

The injectors are usually switched via a ground signal from the computer.

You can check with a noid light.

Try disconnecting the air mass meter, crank and see what happens. I don't even know if this car has one??

If you are getting spark it means the computer is getting a reference signal from the crank position sensor. Can you see an rpm signal at the tach while cranking?

Also check that the injectors have power and check the connection all the way back to the plug at the computer. You should do this before swapping computers because a short to ground to the computer will fry the injector driver.

If you are getting spark I seriously doubt it is a problem with the factory alarm.


Usually the injector harness has it's own connector to the main harness. Check the connections there.

I don't work on Mazda's but the principles should be the same.

[fastfredracing]

Does the guy have a scanner? I would think if the crank position sensor was not outputting a signal, there should be a code set , or, if not, one should be able to view live data, and watch and see if you have that signal during cranking.
I am with Eric on this one, you are either missing a switched 12 v at the injectors or the ecm is not outputting the injector pulse for some reason, probably ckp. A quick test with a noid light will get you pointed in the right direction.

[fastfredracing]

If you need any wiring diagrams, or info, I would be glad to get it to you.

[boosted 89]

Also check to see if the TPS is reading properly, could be reading high which would cause clear flood mode to disable injector pulse.

[1990C4S]

Originally Posted by Laneco

The mechanic .... is a GOOD mechanic.
angela

This statement is suspect, but I will suspend judgment pending final outcome. Seems to me that this is a problem a GOOD mechanic would fix on his own...and he should have already ruled out a lot of these suggestions....but it's the holidays and I will be charitable. Also, I trust your judgment, since you seem to know more about cars than most of us.

[john70t]

Originally Posted by boosted 89

Also check to see if the TPS is reading properly, could be reading high which would cause clear flood mode to disable injector pulse.

Good possibility. Will unplugging it allow starting or is jumpering necessary?

[Joeaksa]

For an $80 buck part, get it and rule that out. Its cheaper than the mechanic's wages for several hours trying to second guess if its working.