98 mpv misfire under load when hot

mazda
mpv

#1

Misfire only under load & only when hot… replaced plugs, wires, cap, fuel filter & pump, coolant temp sensor, air temp sensor. Any help would be appreciated.


#2

Do you have a Check Engine light on? If so, read the code at your nearest auto parts store that will do it for free and let us know what the code reader says.


#3

I would be worried about the ignition coil losing it when it gets too hot.


#4

Thanks for the help, the dist. cap had 6 connectors on top for the 6 plugs thats all, not sure where or how the coil works on this.


#5

The coil feeds the distributor, and yours probably looks like the picture attached. My guess is mostly related to the thing about heat - heat increases resistance & once the coil gets shaky it can break down. Many times auto parts stores can test these if you bring them in. Have you had the codes read? If you have something like a misfire confined to only one or two cylinders I might not suspect the coil.


#6

2 codes from autozone… p0420 Catalyst efficiency below threshold bank 1 and p1345 Manufacturer control ignition system or misfire. A replacement coil at autozone is $100 and they do not test old ones. Never having timed an engine before, would I have to re-time the engine if I put a new coil in or will it time itself automatically? This is my 1st time on the car talk forum and I am so impressed and grateful, thanks again.


#7

Changing the coil should not affect the timing.


#8

You can also go to Autozone’s website, register, plug in the car’s info & get their vehicle repair guides. It gives you the basic steps & specs for checking out the coil - if you have a multimeter. I don’t actually have enough knowledge to say whether this will turn up a heat related problem since the resistance specs are given only for room temp.

Its just that it looks like you’ve been tossing parts at it for a while, so if it were me I’d want to know about the coil before dropping another $100


#9

Thanks for the autozone tip. I replaced those parts trying to fix a similiar problem last year, cranking but not starting when hot. Only way to start when hot was to floor the gas pedal and then turn the key. It was a bad coolant temp. sensor. With 146k on it, those parts were due. If I would have found the coil last year, I would have replaced that also. Time for a new coil, we’ll see what happens. Thanks sooooo much again.


#10

You cannot verify a heat induced coil failure with a multimeter.

But you can with a heat gun.

Coil windings are coated with a varnish-like coating as insulation. As they age and expand and contract due to temperature changes, the insulation can fracture over time, allowing windings to short and affecting the output voltage and the induced waveshape. When the windings then return to ambient, the cracks close up and the pulse returns to normal. You cannot measure this with a multimeter. The best way to verify a heat related problem (assuming one does not have a scope handy) is to induce failure with judicious use of a heatgun.


#11

I replaced the coil today, the problem remains. I can’t believe its the converter, any other tips? If not, I’ll be heading to the dealer.


#12

The only other things I can think of offhand are the igniter (seee link)

http://www.tegger.com/hondafaq/igniter-operation/index.html

and perhaps the crank position sensor. Both can become heat sensitive.

PS: it ain’t the converter.


#13

The dealer tells me I need a new distributor assembly at $500. I already replaced the cap and coil. Does this seem right?


#14

Yes, a new distributor assembly fixed the problem. I guess I was close changing the cap and coil. A new distributor assembly, is that he modulator in there that was causing the problem?

i