diagnosis please?

[rockhead]

hope someone can give some insight to my problem:

a few weeks back i changed out the plugs and wires in my 02 p5 and dropped in a new K&N air filter in hopes of improving gas mileage and bringing back a little more zip to the performance. reading through the board, i got the idea to reset the ECU in order for the car to "start over again" with the fresh sparks. i disconnected the battery, pumped out the brakes, hooked her back up again and all was good.

until....

periodically, not everyday or even very predictably, the car begins to act up. while driving at highway speeds, the front end will begin shaking horribly, enough to vibrate the entire cabin let alone the steering wheel. while that sounds initially like a wheel/suspension problem, it also comes along with a palpable loss in power. RPM will stay up, but the car simply won't respond or accelerate. it feels as if the car wants to stall out, but it doesn't. it may drive like this for a few minutes before it returns to normal. like i said, not constantly and not very predictably, though it seems to happen more when the car is cold and at highway speeds (though i've had it happen in town after driving highway home too).

any ideas? the only thing i can think of before taking it in for service is to try resetting the ECU again.

02 p5
112K miles
all stock except as noted
new timing belt/water pump maybe 9000 miles ago
new plugs/wires

 

[DeadGeneration]

I would reset the ECU again. I notice when I reset the ECU and it's "re-learning" that it decides to play with the fuel ratios and some times dips RPM and get's rough a few times before it gets better. But that's usually at idle not driving at highway speeds.

 

[MazdaSpeeder]

Sounds like maybe a plug wire? Perhaps you're intermittently not getting a spark in one cylinder, hence the vibes and loss of power. I'd check to ensure all are connected well, then maybe you have a defective wire?

 

[TropicalDelight]

sounds to me like a misfire, is the cel on? I would check the gap of the plugs, maybe you need a coil?

 

[rockhead]

no CEL. installed 0.044" gap plugs (NGX iridiums), but i'll double-check to make sure everything's still tight. how would i know if a coil has gone bad?

 

[rockhead]

could i be misfiring a cylinder without the CEL? i retightened the plugs and wires this morning before i left the house but it still acted up. would an issue with the timing behave like this?

 

[rockhead]

i hope someone's still following this:

i think i may have the problem identified to the brakes: after i originally replaced the plugs/wires, i reset the ECU by disconnecting the battery and pumping the brakes to drain the charge. since that time (maybe 3 weeks) i had noticed quite a bit of brake dust on the fronts but didn't think anything of it since the car hadn't been washed in a LONG time! this morning i also noticed that the car would not budge with the brakes released under idle. is it possible that i somehow seized the brakes up when i pumped the charge out of the ECU?

 

[Gasological]

Sounds like a false correlation that the ECU reset caused a brake problem, but you may have a caliper that's hung up.

Again, HUGELY doubt that one caused the other.

 

[Purrfectangel]

To check your coils you need a meter. I'm assuming that it wasn't doing this before you changed things, right? Is this the first time you've used iriduim plugs? Are they long/short reach? Try re-seting the ecu again. If that doesn't work, I would suggest checking your coils, one or more may be bad giving you the spuratic loss in power, check your plugs, try the old ones or another set and see if the same thing happens.

 

[Purrfectangel]

Ceased caliper/s would definitely cause the bad vibration you feel.

 

[rockhead]

Sounds like a false correlation that the ECU reset caused a brake problem, but you may have a caliper that's hung up.

Again, HUGELY doubt that one caused the other.

i agree, i don't think it was the resetting the ECU, but pumping the brakes to do so.. at least that fits the timeline of the problem anyhow.

brake work is probably beyond my DIY level. at least now i have an idea what to tell the shop when i take it in. thanks for all the insight.

 

[Gasological]

I've pumped the brakes to kill the charge after disconnecting the battery plenty of times without causing the caliper to hang up, so I'd bet you're ok to do this again in the future (and anyway, the point is to just *touch* the brake so that the brake LIGHTS come on, no need to hammer it).

If you're really worried about it, turn on the dome light. Same effect, no worries at all.

If you're not sure what's wrong, probably best to take it someplace to get taken care of. Brakes are kinda important, so if you don't trust yourself to fix em, don't do it yourself. But DO hang around the shop (after asking nicely) so you can learn. Some folks don't like it when you peer over their shoulder, but you might find yourself somebody who likes playing teacher.



The whole thing could be a shorting ignition wires issue, especially if you've just moved them around for the first time in a long time, but that would likely throw a misfire CEL, especially if you were getting violent problems at highway speeds.

 

[rockhead]

i quickly talked myself out of playing with the brakes and called the shop to take it in tomorrow. no doubt that's beyond me at this point. i'm keeping my fingers crossed that that's what the problem is at this point. sounds like an easier/cheaper fix than ignition/timing.

thanks again!

 

[rockhead]

2 new front calipers getting installed now. hopefully that's the only issue!