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740 possible spark problem?

Mbeas96

Member
Joined
Aug 7, 2017
Location
Charleston
1990 740 lh2.4, aw71, chipped ezk and wasted spark using miata powerstage and bosch coil pack (out of a land rover discovery), recently non chipped 9xx fuel ecu which fixed my random rich stumble/misfire, 16t at 12psi, npr intercooler, t5 injectors and resistor pack bypass, do88 turbo inlet hose

On my 25 mile trip home from work, all was fine, passed a couple people on the 2 lane highway and not a hiccup. All the way until about 2 miles away form my house, i floor it from a stop and it is taking off alot slower than usual building boost and as soon as it hits 10psi it starts bucking and misfired right before i let off. Very suprised i let it cool for about a mile and a half and tried to slowly build boost in a higher gear. It is very inconsistent power and bucks alot. AFR's on the guage show it goes from 12s to 14s and back again.

Thinking i was experiencing some sort of spark blowout, I made it home, let it cool, and reseated the plug wires on the plugs and onto the coil pack. Also made sure there were no tears in the wires. This time, pulling out the driveway switching from reverse to nuetral the afrs went full lean (17+) and the rpm guage was barely above 0, car was shaking and about to turn off, i tapped the gas and the problem stopped. idle normal and made it to the end of my road. Went to turn onto the highway and same thing. Shaky car, lean reading and dying rpms.

Let off the gas and got bakc on it and it pulled out into traffic fine. But as soon as i tried to get into boost, afr's start going crazy, bucking and farting start. Went half a mile to a gas station, turned it off, started fine, idled fine, pulled thru the parking lot fine, but again more than like a quarter throttle and things start to go wrong.

No CEL, no codes on sockets 2 or 6. rsr and harness and fuel pump relay replaced around 6 months ago, plugs and wires around a year old. Not to sure where i should start checking things out. Usually when things go wrong with this car it pops a cel code or something to get me started...
 
I would start with easy stuff...remove and examine all intercooler piping, or pressure test it, I had a similar problem, checked all the tubing, and swore up and down there was no leaks...turns out I did, output of intercooler, on the bottom of the hose. The only way I found it was a tiny bit of oil had started to come out an it got on my hand.

I'm sure you thought of leaks, but look super carefully at every one.
 
In all honesty...rather than a glance around I didn't check for leaks. If it is a tear, I have a suspect in mind...I'll look more into it tomorrow, if none found I'll set up the pressure tester and see what happens.
 
Pressure tested today after work and sprayed soapy water everywhere. I found a small leak, also by the output of the intercooler. Got that pretty much squared away. The problem has also seem to have gotten worse? over the last 24 hours. Originally i could drive, just not hit the gas very hard or things would start going bad. Now, i start the car and it's not driveable. It's shuttering, idling low and running lean. It will not rev, if you hold your foot on the gas it'll slowly rise but sounds like it's being choked or something.

I tried unplugging the AMM and the engine shut off, so that seems to be working. I'm tempted to swap coil packs to my spare, or swap ecu's back to the 5xx

Here is a little clip of earlier after the pressure test...

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Will it idle without assistance? If so, try wiggling all the wires that you can get to. It sounds like a bad connection, or a short, especially since it seemed to show up with vibration.
 
It'll idle, at about 200-300 rpm. I'll go wiggle everything in sight. Will report back

And not currently, my last one failed...

I'll break out the multimeter and see if I can test it
 
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Did the jiggle test on the spark plug wires, coil pack wires, everything associated with the wasted spark that's in the engine bay, and the rsr since I wired that in too. Nothing changes...

Pulled the spark plugs, a little oily from my leaky cam cover and number 3 is a little dark...but I don't think these would be my culprit. The wires are only a year old and seem like the sheath is still in great condition. They were however all gapped to around .42 so I put them back down to .29-.30 and reinstalled. No change in the cars symptoms.

Can't find the multimeter atm...so no info on that right now

The car sounds different. Like more throaty or something..maybe cause it's just running bad but could a clogged cat be causing this nonsense? It's the original I assume.

Also checked the fuel pressure. Engine off key on pressure is 38lbs. Engine idling pressure is 38-40. Trying to rev, probably only getting to 1200 or so and it's going from 35-40. No gas in the fpr line.

Edit. Just swapped in the 560 ecu and no change. Coil pack will be next when I can find it..
 
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My brother gave me another spare AMM to try.

Plus we were talking about spark plug 3 that was dark compared to the others. Maybe that cylinder is being flooded with fuel? So today I'll take off the fuel rail and see about rigging something up to test the spray patterns of the t5 injectors. Hopefully when needed I have one extra from when I bought the set I could throw in.

Also for some cheap insurance I'll grab a new set of copper ngks and gap them down to around .022 instead of .03.

I wish I knew whether it is spark or fuel related. Anybody know how to figure that out? :D
 
Backfiring out the intake when trying to start? Never heard that before...

Swapped AMM and coil pack. Didn't do it. The car seems to have gotten worse. Won't stay running anymore. Won't fire on the first try. It will eventually after a few cranks but idles so low that it eventually shuts off.

The car has like an 1/8th of a tank right now. Is it possible the intake pump is shot and it's running like this because the gas is to low for the main pump? I opened the access panel and it looks like the original, pretty crusty. But it primes with the key...Idk

Still no codes from the box
Just thought about it...it's probably running worse when it's cold, then I've also reset the computers. So maybe it's still the same when it's warm. Just can't get it warmed up because it won't stay running.

Swapped FP relay to new one I had. No changesssss
 
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If your in tank pump is dead and the fuel level is less than 1/4 the main (and now only) pump ttry to suck the fuel heavy. This will produce under pressure. The fuel will start to bubble/foam. A injected engine won't run with fuel-foam. Result: to lean (backfire to the inlet side)
The ECU wants to compensate, no chance.

First to check: in tank pump.
Maybe your car is with the schrader-valve in front of the main pump. Otherwise open the fuel line between the in tank and the main pump.
If the in tank pump is dead/clocked fill up the fuel cell.
Or swap the in tank pump first.
Maybe a wallbro in tank as a main pump will be your solution.
Good luck, Kay
 
The intake pump definitely primes with the key. Not positive that it's on while running. But my thought were maybe a hose on the inside of the tank is torn or something like that?

Either way, I put around 2 gallons in and still the car wont start...not sure why it has progressively gotten worse. More gas?

I guess starter fluid next? I want to see the injectors spray pattern but the fluid will probably happen tomorrow.
 
Okay so, put original coil pack back on since I didn't yesterday and still no start. It was trying to fire but wouldn't stay running. Thinking back to my thought of injector 3 being stuck open, I unplugged it and nothing changed. So I sprayed some starter fluid directly into the intake and nada. Didn't even try to fire.

Took the plugs out and all of them are soaked with gas. I have new ones but I broke a plug wire pulling it off...I'll track some down tomorrow hopefully.

How do you test spark with a coil pack? Either they aren't firing or the injectors are just unloading the fuel...
 
Test the same way as you would without coil packs? lay plug on something steel on the motor and see if it sparks? I have never tried it with coil packs, seems like the question is why wouldn't they be working? I don't have experience with your setup, the only time I had coil pack problems was with my V50T5 and replacing them just fixed it.
 
Okay, makes sense lol. Never had to do it before so just making sure.

And no idea...taking stabs at the dark currently. Plus the igniter came from a 90's miata in a junkyard...Really I'm pinning things that I have personally messed with, even though everything is soldered, heatshrinked and zip tied in place.

Grabbed some plug wires from my brothers car while his sits, I'll throw the new plugs in tomorrow and see if they get coated with fuel. If so, connectors off injectors and see if the plugs still get coated. Rail off to check injector spray is my last idea.
 
Gapped the new plugs to .26 and car started. Ran like dog ****. But started. Same symptoms as before, very low shaky idle. Barely runs and chokes more with throttle. Pulled the plugs and they are wet and reek of gas. About 10 seconds of cranking and 10-15 of running.

Let them dry and they didn't stink, put thme in and unplug the injectors and crank. They smell faintly of gas again.. it's gotten dark and I'll wait till tomorrow to pull the rail.

Either gas was still in the cylinders after about 30 minutes or they are leaking. I hope they are leaking
 
https://youtu.be/k3o8gIU8-o4

Took the rail off..problem is Idk what I'm looking for. Besides them just pouring out gas. They definitely have a pulse. Watch in 480 if you can, you can actually see it

I feel like it looks how it should. They are pulsing, no one sprays significantly more than another. Maybe it is a spark issue
 
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If your in tank pump is dead and the fuel level is less than 1/4 the main (and now only) pump ttry to suck the fuel heavy. This will produce under pressure. The fuel will start to bubble/foam. A injected engine won't run with fuel-foam. Result: to lean (backfire to the inlet side)
The ECU wants to compensate, no chance.

First to check: in tank pump.
Maybe your car is with the schrader-valve in front of the main pump. Otherwise open the fuel line between the in tank and the main pump.
If the in tank pump is dead/clocked fill up the fuel cell.
Or swap the in tank pump first.
Maybe a wallbro in tank as a main pump will be your solution.
Good luck, Kay

Good advice from Kay!

That intank pump is essential to feed the chassis pump, critical at low petrol level in the tank.

Before overhauling your ignition system, can you go back to a timing light and see if can see proper timing for the #1 spark? I've had some luck depending upon the engine when cranking, but proper timing of the spark is probably more important that "spark quality" itself.

Edit - is this vehicle still LH2.4?
 
It is still lh2.4. Which sets the timing itself, correct?

I realize this thread is getting long and alot of people don't read thru everything so here's a recap of whats happened so far...

740 lh2.4 specs in first post
Very low shaky idle, dies with throttle, was not this bad when it first occurred during WOT pull driving home from work.

Pressure tested system- checks okay
Checked fuel pressure - 38-40lbs
Fpr is not leaking out of vacuum line
Changed plugs
gapped new Plugs to .26
Changed a couple questionable plug wires
Swapped coil pack to spare that was working a few months ago...
Unplugged AMM car died, swapped AMM and no change in symptoms
Intake and in line fuel pumps prime with key
Changed fuel pump relay
Disconnected cleaned and reconnected rsr
Finally pulled rail last night and all injectors pulse fuel and look okay. Plugs still smell like gas after a few cranks
 
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