MadDog

I need a little assistance. My dedicated track car is having a power loss issue and I have an event coming up at TWS in 2 weeks, that I can not miss.

At the end of my last track day, I started having power loss starting between 5,500 and 6k RPM. After 6k, it is pretty weak. This has never been a problem before and always pulled hard all the way to redline. I'm running 10 psi of boost and the boost never drops off when the power loss is happening.

The behavior is very repeatable and does not have to be hot to happen. It behaves the same when it is just warm. After it happens, I can lift the throttle and let it drop back down to 4k RPM, go to WOT, immediately pulls hard and is at 5,500 in a blink. At this point, it drops off again. I don't think it is a fuel filter problem because there is such a significant difference between 5k & 5,500 even when I give it a chance to catch up on the fuel delivery before hitting it.

My first guess was plugs. I just replaced them and it had no effect. They have been in for 2 seasons and they looked clean (not carbon-ed-up). I'm using NGK BKR6ES gapped to 0.032", which is exactly what I have been running for the past 2 years with no issues (and recommended by BEGI). I don't know the power at the wheels, but I'm guessing it is 180 to 200 WHP at 10 PSI (when running correctly).

The wires are relatively new, but I'm still running the original OEM coils. My next guess is the coils. My question is whether bad coils can cause this behavior? Where else should I look?

I"m running a parts bag turbo setup with a Nissan compressor and a Garrett turbine with XEDE engine management, all installed by BEGI down in San Antonio.

Any help is much appreciated from the Miata masters. I'm an old Vette wrench myself.

MadDog

SKMetalworks

iTrader: (7)

What do your AFR's say when this stumble happens?

MadDog

Good question, but unfortunately I don't have a quick answer yet. My wideband overheats pretty quickly even with a pretty large copper heat sink on it. By the time I looked at it this evening, it was already showing the overheated code. I'll take it for another drive tomorrow and see if I can read the gauge for one run before the wideband starts throwing overtemp errors. It used to only give me the overtemp errors after about 20 minutes of hard running at the track, but now it will even happen just driving around side streets on a moderate test drive. I'm guessing the wideband O2 sensor may be giving up the ghost.

When I'll try and get a measurement tomorrow. Thanks for the response.

MadDog

SKMetalworks

iTrader: (7)

Not sure if its possible with the xcede but i know the megasquirt maps can get corrupted some times and change things. Possibly try re-burning the map ( just an thought )?

MadDog

Hmmm. Interesting. I should call the BEGI guys tomorrow since they are the XEDE experts, and see if they have seen that. Thanks. You're right, though. I should see that on the AFR meter if it is happening.

gospeed81

iTrader: (51)

Spark can be finicky. I was right at your power level and had the same issue. It had been fine that way for months, but when it got cold and the air was a little denser I was chasing around blow-out.

Gapping down to .020" helped at first...but I eventually went to COPS. I don't know if my old coils may have been going out...but I know the Toyota ones sure as hell fixed the problem. It was nice just throwing in stock-gapped plugs and not worrying about it.

kevin02

I had a similiar problem recently....changed plugs, coil, wires all at different times...wires were the culprit.....cheap to find out if its the wires...

messiahx

iTrader: (2)

I would bet it's spark blowout. The first time I dove my car boosted in winter (stored it in summer, picked it up in December), it'd get up to about 5k rpm fine and then blowout would occur. Gapping to .020" fixed it. But you'll know if it's blowout -- you can hear and feel it sputter.

MadDog

Okay, here's an update. I talked to the guys at BEGi to see how likely it was that the fuel map was corrupt, since they are the experts on the XEDE system. Tim said it is unlikely that it would run smooth and strong up to 5,500 and then start losing power if the fuel map was corrupt. It was more likely that it would run badly everywhere if this was the case, which makes sense to me. Tim thought the plug gap was fine for my setup and recommended to check these in order, as well as to check the AFR as SKMetalworks suggested.

1. Spark Plugs
2. Air Filter
3. Dirty MAF (from K&N oil and dust)
4. Plug wires

Since I replaced the plugs yesterday, I checked the air filter. To my amazement, it was really dirty. It never crossed my mind to check it because K&N recommends cleaning every 50k miles and mine only has 3k miles on it, although it is all track miles. Note to self... 3k track miles equals 50k street miles.

I cleaned the air filter and cleaned the MAF sensor, then took it for a test drive around the block (and got a #$$%#$@#@%% ticket for outdated inspection sticker... but I digress). Problem is unchanged and I was able to watch the AFR before it overheated. To do this, I went to 2k RPM in 2nd gear and went to WOT. I kept some loading with the brakes so I could let it accelerate just slow enough that I could get an accurate read on the AFR (poor mans logging). Normally it is rich at all times at WOT. What I found was that it was rich as expected (~12) up to about 5k RPM. Between 5k and 5,500 it pegs the meter on the lean side (20+). This is NOT GOOD, but I have no idea what could have changed other than the fuel map. Since I ran it a couple of sessions like this at my last event, I hope I didn't hurt anything. It may also explain why my wideband is overheating now (which is why I didn't see it running lean at the track).

One more thing of note. The reason I had to cut my event short was that my exhaust header cracked where the wastegate was attached from vibration. This caused an exhaust leak that boiled my clutch fluid. This happened around the same time I started losing power and I assumed I was losing power due to the exhaust leak before the turbo. I was surprised to find the lack of power still there after welding up the crack in the header. The only mechanical damage that could be caused by the exhaust leak that I could think of, was a burned valve in cyl #4, but this is unlikely since it runs strong with no missing below 5,500 RPM. Also it would not explain the lean condition.

Could this be a fuel pressure problem (i.e. the fuel map says more fuel by longer injector duty cycle, but the pressure has dropped so less fuel is delivered)? What is the fuel pressure supposed to be and is there an easy place to check it? I understand diesel truck engines pretty well, but I'm not that familiar with the Miata setup, especially one with a piggyback ECU.

Thoughts?

MadDog

MadDog

For those interested in building a fuel system pressure test rig to test your fuel pressure at the pump, filter, regulator and at the fuel rail, I’ll share my experience. The basic idea is to be able to disassemble any quick high pressure quick-disconnect fitting in the system and insert your test rig in between the male and female quick disconnect fittings.

I went to about 5 auto parts stores looking for a female quick disconnect to make my rig since it was the most difficult piece to find. Autozone was the only one who sold one. They sell a fuel line Quick Disconnect Nylon Fuel Line Repair Kit. It is part number 800-057 by Dorman. Ignore that it says it is for a GM/Chysler. It is the right one. It comes with a 5/16” quick disconnect release tool, 5/16" hard plastic hose with a female quick disconnect elbow and a barbed union nipple specially designed to go into the hard plastic lines. Once you have this, you still need a 5/16” male quick disconnect nipple, 100 psi pressure gauge , 5/16” tee and some 5/16” rubber fuel line, assorted clamps and whatever else you need to connect to your selected 100 psi gauge. To get the 5/16” male quick disconnect, buy a new fuel filter for your car and install it. Take the old filter and cut off one of the male nipples. Make it as long as possible because you will need every inch.

You basically now have what you need to assemble a “Tee” that you can insert into the middle of the quick disconnect fittings. One side of the Tee is the male nipple and the other side is the female fitting and the 3rd part of the Tee has your 100 psi gauge. Now you can start testing fuel pressures.


If you want to test for fuel starvation issues, it is best to start as close to the fuel injectors as you can get and work your way back to the tank. If you have rated pressure inside the fuel rail that feeds the injector at all times (including max load which is WOT at max RPM), you can rule out the entire fuel delivery system and focus on the injectors, ECU, programming, etc. If anyone tells you that you can have rated pressure in the fuel rail and still have a "fuel pump volume problem", tell them to enroll in some physics classes. Trust me, if your pressure never drops below whatever pressure is rated to be delivered the injectors, the system is delivering plenty of volume. If it can't, the pressure drops.

To test the fuel rail pressure, find the hard plastic fuel line that goes from the fuel rail (under the intake) to the fuel "pulsation damper", which is mounted on the RH inner fender. Disconnect this quick disconnect fitting and instert your test rig.

MadDog

I still have power dropping off between 5k and 5,500 RPM and the AFR goes from rich to very lean at the same time.

Okay, I got a 99 Mazda Miata factory service manual and spent some quality time with it.

The spec in the shop manual for the 99 Miata 1.8L fuel rail pressure is 53 to 61 psi (fuel rail is called "fuel distributor" in Mazda speak). I inserted a test rig at the output of the fuel "pulsation damper" on the RH fender at the connection to the hard line going to the fuel rail under the intake manifold.

If I test it sitting in the garage just like they describe in the shop manual with the key on, engine off and the jumper wire in the data link connector to turn on the fuel pump. I get 61 psi. This is the only pressure test that the shop manual describes at the fuel rail. If I were reading a Ford or GM shop manual, it would tell you to test the fuel pressure on the road at max load under the low power failing conditions to check for fuel pressure dropping and causing a fuel starvation issue. Evidentially Mazda does not impart that little nugget to their technicians and lets them figure it out for themselves.

Since they give a spec pressure range of 53 to 61 psi, I assume that this spec is for all conditions, including max load. However, this is not completely clear. If anyone has ever tested this on a correctly functioning car, please speak up. I’m assuming a drop in pressure below this spec at full load is a failure that needs to be fixed.

When I take it out on the road and put it under load at WOT, my fuel pressure is NOT within the specified range. If I start at 2k RPM at WOT, the pressure is 61 and starts dropping rapidly at 4k RPM and by 5k RPM it is down to 38 to 40 PSI. Since my engine power flattens out starting between 5k and 5,500 RPM at WOT and my AFR goes from rich to very lean at the same time, my logic says that the lean condition could be due to a drop in fuel pressure because the fuel system can not deliver enough fuel to the injectors starting around 4k RPM. If the ECU fuel map is calling for a certain amount of fuel for the given condition, this translates into keeping the injector open for X milliseconds (duty cycle).

The fuel maps assume that the minimum rated fuel pressure is always in the injectors to deliver the correct amount of fuel. If the pressure is low, then we have a lean condition? Take it to the extreme. The ECU calls for the injectors to stay open for 2 ms expecting 60 psi of fuel pressure spraying through the nossals and suppose you only have 5 psi in the fuel rail. Same air, less fuel, lean condition. Is my logic sound? Anyone disagree?

fooger03

iTrader: (2)

Stock fuel pump? Get yourself a Walbro 190 HP and be done with it. Fuel pressure should not drop with an increase in RPM, it sounds like yours is on its last leg. The fuel pump should be pumping the same volume of fuel at all RPMs, and the pressure should be constant across those RPMs. There is a fuel return hose and a fuel pressure regulator in line somewhere near your fuel tank to return the excess fuel to the fuel tank under lower fuel consumption. Lean burn is likely the cause of your overheating O2 sensor. Also, if your timing is retarded, it would cause higher temps at your O2 sensor and a loss in power, but it would be across the whole map and would possibly cause overboost.

miatauser884

iTrader: (11)

Everyone has probably thought of this, but my own experience is driving me to post.

What about mild clutch slipping? This happened to me on the dyno. It was intermittent at first and we thought the spark was dropping out.

I then had a track day, where it started mildly (the loss was smooth, not on/off). At first I though I broke the tires loose. As the day progressed it happened more and more, then the loss was more dramatic. It became apparent that it was the clutch.

Ben

iTrader: (33)

Yes if your fuel pump can't keep up 60 psi at WOT, it needs to be replaced. I recommend the Walbro 255 lph HP for your car.

MadDog

Thanks guys. That was my logic, but I wanted to confirm. I just ordered a Walbro 255 Lph pump. It should get here Thur evening. I'll install it and recheck the pressure and see if that fixed my lean condition.

MadDog

MadDog

Good question, but it is not the clutch in my case. Sometimes it can be tricky to tell if your clutch is slipping, as you said.

The best way to tell if the clutch is slipping is this. If you have a safe place to do it, put it in an high gear like 4th in the power band and see if the RPMs increase at the same rate as the speed. The higher the gear, the more stress is on the clutch (at steady state). With the clutch out, it will have more tendency to slip in 5th than 1st at the same engine power level. If the RPMs go up and the car is not accelerating, the clutch is slipping. You can also get your tuner to watch for this on the dyno.

In my case, I have a relatively new (heavy duty) clutch in good condition and I never hammer it (roadracing only). Also, slippage would not explain the extremely lean condition that happens simultaneously with the low power failure. I'm thinking fuel pump.

MadDog

fooger03

iTrader: (2)

If you ordered the 255lph, do you have an aftermarket FPR? I'm not entirely sure on the '99-00 "short-return" fuel system, but the NA stock FPR (and quite likely the stock NB FPR) is unable to flow enough fuel to handle the 255lph. If you ordered the 255lph HP (high pressure), your problem may be further exaggerated. The reason I suggested the 190lph HP is that the stock FPR can handle the flow, and the "HP" identifier ensures that there isn't a drop in pressure at higher flow rates.

Ben

iTrader: (33)

The NB fuel system is completely different than the NA. The best pump for the NB is the 255 lph HP. The NA's FPR can be overwhelmed by a large pump like the 255, so I recommend a 190 lph pump for the 90-97 cars.

MadDog

Sorry fooger, I forgot to respond to that. I talked to the guys at TREperformance about the fuel pump size and the FPR. They said the same thing Ben said. The 255lph HP is the right one for the NB cars and the FPR in the NB is heavier-duty and should be able to handle the 255lph HP with no problem. We will soon see if this is correct.

MadDog

fooger03

iTrader: (2)

Awesome! Like I said, wasn't entirely sure on the NB - learn something new every day!