Overboosting?

[MattsMSP]

I've been having some misfiring going on with my MSP to the point of where the CEL flashes at me. I went and scanned it at a local parts store with 'random misfire' appearing. I ran it to the Mazda dealer today to see if they could be any more specific, and I just happened to notice something funny on the way there, which may be the cause of the misfiring...?

I have a narrowband AFR gauge (yes, I know, it'll get replaced soon (cheers2)), but while in boost, it holds steady. It usually hovers around 11.5 while in boost, but I noticed today that as the RPMs rise, the AFR slides to 19.0 - this is around half throttle. I didn't go WOT, but I mentioned this to the tech and he confirmed it with his laptop and going for a drive. They said what appears to be happening is the computer is seeing an over-boost and slowly pulling away fuel. My boost gauge peaks at 8.5psi (so does theirs), so, if this is really the case, why does my car pull fuel at 8.5psi while most others say they don't have that issue until 10psi-ish?

So far the car has forged rods/pistons with a 20overbore, new crank, oil pump, cams, valve seals, and a bunch of other misc. things along with a 2.5in turbo back exhaust with high-flow cat. I don't want to do anymore 'fun' work to the vehicle though until I get this sorted out. According to them, at idle, all the fuel trims/pressures are exactly where they're supposed to be - this is the only thing they can come up with. He said getting a piggyback (in the works) would alleviate the problem, but I want to make sure that the 'overboost' scenario is really what's causing the problems.

Does anyone have any other suggestions as to what may be happening? If you want some more info. from me, let me know. Thanks!

 

[tweety2300]

If you have a narrow band it won't show your exact afr. It will just move around from lean to rich depending on the voltage the o2 sensor sends to the ecu.

It should never be at 19 unless you're coasting, what turbo are you using an what are you tuning with? With all those mods you need something

 

[Spiced MSP]

replace and/or gap your spark plugs. Use NGK only. Replace your coil packs and wires. Check for vac leaks first though. Vac leaks can throw a random misfire

 

[MattsMSP]

If you have a narrow band it won't show your exact afr. It will just move around from lean to rich depending on the voltage the o2 sensor sends to the ecu.

It should never be at 19 unless you're coasting, what turbo are you using an what are you tuning with? With all those mods you need something

Hi Tweety2300. My gauge bounces like a 2 year old under normal driving, but holds steady once a load is placed on the motor. I didn't expect it to match the reading the tech got today, but it surprised me when he said it did. I'm running with the t25 and no tuning as of yet, but want to get something this spring when it gets warm enough to work on the car outside.

replace and/or gap your spark plugs. Use NGK only. Replace your coil packs and wires. Check for vac leaks first though. Vac leaks can throw a random misfire

Hi Spiced MSP. The plugs were replaced this past October with NGKs and were re-gapped at the beginning of December. Nobody has found a vac leak yet, but if that pops up it'll get fixed immediately. I had thought there was a problem with the coils in December, when I re-gapped the plugs, and marked the one that read a goofy impedance. I asked about the coils today and was told that if it were a coil issue, the misfire would be more consistent than it currently is and the AFR would go rich. Is this true? I personally have never experienced a bad coil. Right now, when/if it misses, it sounds like 2 cylinders go down, which is why I asked about the coils. It has only shown the symptoms 3 times in the last 400 miles or so and has happened while pulling away from a stop in town as well as cruising at 65mph down the freeway. In order to remedy it, I had to stop the car and restart it on two occasions. The third time it fixed itself.

 

[Sport23]

Hi Tweety2300. My gauge bounces like a 2 year old under normal driving, but holds steady once a load is placed on the motor. I didn't expect it to match the reading the tech got today, but it surprised me when he said it did. I'm running with the t25 and no tuning as of yet, but want to get something this spring when it gets warm enough to work on the car outside.



Hi Spiced MSP. The plugs were replaced this past October with NGKs and were re-gapped at the beginning of December. Nobody has found a vac leak yet, but if that pops up it'll get fixed immediately. I had thought there was a problem with the coils in December, when I re-gapped the plugs, and marked the one that read a goofy impedance. I asked about the coils today and was told that if it were a coil issue, the misfire would be more consistent than it currently is and the AFR would go rich. Is this true? I personally have never experienced a bad coil. Right now, when/if it misses, it sounds like 2 cylinders go down, which is why I asked about the coils. It has only shown the symptoms 3 times in the last 400 miles or so and has happened while pulling away from a stop in town as well as cruising at 65mph down the freeway. In order to remedy it, I had to stop the car and restart it on two occasions. The third time it fixed itself.

If the misfire is bad enough (it sounds like it is) that unburnt fuel is getting dumped into the exhaust then your AFRs will read lean not rich. 02 sensors only read gaseous hydrocarbons; liquid fuel in the fuel in the exhaust will not register at all. I had this issue with my MSP. In my case the spark was jumping from the coil, past the insulator, to the base of the spark plug and grounding out thru the cylinder head. In vacuum it was fine but in boost it would misfire and stumble badly.

The two plugs I had issues with were the ones directly under the coils; the hard plastic boots on the base of the coils were not doing there job. It left carbon tracks down the plug insulator and was so bad that it etched the ceramic (I kept blaming it on the MSPs overly rich tune until my SSafc failed to fix the problem.) I've heard of other people looking at the coils in the dark and seeing the spark jump from the base of the coil to the cylinder head.

I had some FP coils in my garage so I did the conversion, with new plugs and wires, as soon as I diagnosed the problem. I suspect that if I had replaced the plastic boots on the coil packs and my plugs that it would have fixed my issue also. Btw, I *never* got a single misfire code!

 

[MattsMSP]

To give you an update, I got ahold of some coils and put them in today, then reset the codes. Coils came with new boots for the plugs directly under them. The car still goes crazy lean when in boost as RPMs increase, but the misfire code never showed. I didn't replace the wires yet, but I'll look at that stuff tomorrow.

 

[MattsMSP]

Here's where I'm at: I've replaced the coils and all the plugs, gapped to 0.028". The old plugs don't have any carbon trails on the ceramic, nor do they have any cracks. The car runs nice and smooth, but it still goes lean as the RPMs climb under boost. Does anyone have thoughts on what I should look at next?

Here's a pic of of the plug from Cyl. #1:

 

[MattsMSP]

To update this, I've now found that regardless what the boost pressure is, it goes lean around 4k RPM in any gear. It does this consistently anywhere from 1psi to 8.5psi and the throttle position doesn't seem to affect it. The entire intake system has been checked for loose clamps and cracked pipes, but I found nothing - MAF was cleaned at this time.

Anyone have suggestions on what to check next?

 

[Sport23]

I would guess that you have some very negative long term fuel trims. Have a look at your short and long term fuel trims while cruising and your long term fuel trim in boost.