10 replies to this topic

[SlowPoke]

I need help. I have a 1997 Miata that I have been having problems with for a year. I have replaced the cam sensor, the crank sensor, multiple coil packs, the wiring harness to the coil packs and fuel injectors, the ECU, the MAF, electrical relays, fuel pump, fuel filter, gas tank vent and associated plumbing, gas cap, plug wires, spark plugs, and pretty much every other electrical component associated with the engine. I have cleaned every ground connection on the car, sprayed contact cleaner on most of the connectors, and added an extra ground cable from the engine to the chassis. I have bypassed the factory wiring to the fuel pump and have it on a toggle switch. The car runs fine for about 15 to 20 minutes and then the engine goes off song. It feels like it is running on two cylinders or running out of gas. After it sits for 2 to 3 minutes in the pits it cranks and runs fine again. The current problem is that after 15 to 20 minutes on the track it goes off song and both coil packs have a small hole in the side wall with a silicone looking material leaking out. It does not crank and run fine in this condition. Has anyone had a similar problem and have a solution? I am trying to run this car in ITA.

 

Thanks,

 

Gregg

[Steve Scheifler]

The coils are leaking out a hole in the side? And this has happened to new replacement coils? It sounded familiar until you got there. Doe the coil get hot? Have you done the impedance check of the coil per the FSM when cold and again when this problem occurs?

I haven't encountered a situation where coils are cooked but have you checked alternator output voltage? Seems if it were high the battery would suffer, especially if you run the stock type, but worth checking.

[SlowPoke]

Well......everything is hot when I come off the track so I'm not sure what temperature you would define as being too hot for the coils. I have checked the coil packs with a multi-meter and the coils are shot. Basically no reading across the primary side. The book gives no data for the secondary side on the 97 Miata. All this is with ref. to the coils with the hole in the side. When the coils are in spec the car runs; when the car quits running the coils are shot. The question is, what is causiing the coils the fry? It has been suggested that I rewire the entire car.

[Steve Scheifler]

By cooking I meant the possibility that your alternator is putting out too much voltage. I don't see the coil being the first thing to suffer, or even know if it would suffer at all, but grasping at possibilities until someone chimes in with a better idea. When you check voltage be sure to pick the revs up well above idle. You should see 14 +\- 0.5 VDC.

[SlowPoke]

I have a voltmeter in the car and it typically stays in the 13.5 to 14 range.

[davew]

You have a bad ground connection under the throttle body.

 

I have seen this several times. It only happens with NA 1.8 cars. Usually takes out the coil pack. Some times the ECU and sometimes both.

 

If the coil has the little silicone snake coming out, it is junk. Continued usage will definatey take out the ECU.

 

I am 99% sure on this.

dave

[Diller]

I have chased my tail with what I thought was a repeat failure of a cam sensor. Basically the car would be fine for a few laps and then cut out during the corners and come back to life. This went on for a while before the car wouldn't start at all. I checked the voltage at the cam sensor and found that one of the wires with the 5V supply was broken but the insulation was still good. The copper had broken but the wire looked fine. 

 

I didn't have the coil pack issue but I can see that if your cam sensor is sending bad signals that the coil may be firing more than it should. I would check the continuity of the wires going to and from the cam sensor.

 

[Begin_Soapbox]

 

Now that these cars are getting 25+ years old, is it time we start to accept that the stock wiring harnesses are no longer suitable for racing use? We already made the allowance for the cam sensor to have a soldered pigtail to a different connector so that is going in the right direction but that only addresses one piece of a systemic issue. If I were king for a day, I would allow all mission critical items (fuel pump, cam sensor, ignition coils, etc) to have a separate wiring harness.

 

[end_Soapbox]

[SlowPoke]

I have had several coil packs with the silicone snakes. Car runs fine for 15 to 20 minutes after I replace the coil packs. Once I replace the coil pack again and correct the ground issue under the throttle body, how will I know if the ECU is damaged? If the car starts and runs is the ECU OK?

 

Thanks.

[davew]

I have had several coil packs with the silicone snakes. Car runs fine for 15 to 20 minutes after I replace the coil packs. Once I replace the coil pack again and correct the ground issue under the throttle body, how will I know if the ECU is damaged? If the car starts and runs is the ECU OK?

 

Thanks.

 

Pretty much that is all you can do. next time you are on a dyno, bring a spare ecu for comparison.

 

dave

[Keith Novak]

Although it sounds like it's not the issue in this case, one thing it reminded me of is a kill-switch issue.  Not everyone wires them the same and when the switch goes bad, many strange electrical issues can occur.  Symptoms when mine just went out included:

• Car died on track while I was sorting out a fuel pump issue. (Thought it was pump related).  Would not restart.  Restarted fine after being towed back in and allowed to cool.
• Car would run fine on the track for about 20 minutes and then start to die like it was running out of gas
• Transponder died.  Light on but no signal.
• Battery wouldn't charge.  Alternator measured it was putting out 35-40 volts while voltage was steadily dropping at the battery.
• Electrical components started getting abnormally hot.

I bypassed the kill-switch and the voltage went back to normal then replaced it and no more problems. 

Just a diagnosing tip for next time to take one wild-card out of the equation.

[SlowPoke]

Many thanks to Dave Wheeler. I found a ground wire from the post under the throttle body that had zero conductivity. I cut out that section of wire and replaced it and did a bunch of other work on the wire harness and ran my first race in over a year a couple weeks ago. Qualified 8th in ITA and finished 2nd. It was a good weekend. Thanks Dave.