28 replies to this topic

[DrDomm]

As I plan my return from hibernation, I'm stuck with a confusing problem that plagued me all of last season.  Last year, I ran my 2000 I built with some of the components (including engine) from the '99 I wrecked in 2014.  Just a quick summary of the drama with that car...Early in the season I had a bad misfire that was likely due to the fact that I had the cam timing wrong.  After having the cam timing corrected I got rid of the misfire, but then had an issue with the engine running lean with decreased power.  We had to turn the fuel pressure up to 60+ psi to make max power, and were still a bit lean.

 

I posted this back in May...http://mazdaracers.c...+fuel +pressure, but never really figured out the problem.

 

Unfortunately, we determined that some of the valves were damaged from cam timing being off.  I got that repaired, and was hopeful the above problem would be resolved.  Though we dyno'd the car at the same power this motor made in the '99, we had to do it with over 60psi of fuel pressure...and the 5500rpm AFR was over 13.  Prior to this I also swapped fuel pumps (not new) and installed rebuilt injectors (from Dave W.).  From there, I just ran a few races, and did the Runoffs.  I did swap in another MAF meter, but no difference.

 

So, I've got a car that needs over 60psi to get maximum horsepower with the AFR about 13.2 at 5500rpm. I get great fuel mileage (but don't run enduros).  At Mid-Ohio I averaged about 12.3mpg.

 

I've been studying fuel systems, but nothing jumps out at me.  The car starts fine, runs fine, but clearly something is wrong.  I data log the fuel pressure in the loop of hose that comes from the fuel line into the engine bay, before it gets to the pulse dampener thing.  I get a drop of about 1.5psi from 5000rpm to redline.

 

Any help is appreciated.

[Rob Burgoon]

Have you tried a different ECU?

[chris haldeman]

I'm with rob on this one. If the original car was purchased used as a race car there is a strong chance it has a modified Ecu. Old school modified ecu's required 58-62 psi as that is what the factory regulator produced.Another issue would be that the Ecu also has timing advance built into it and if you are setting the timing to 15 base you will also be getting detonation that is costing power and possible valve damage

[ChrisA]

Have you verified the pressure sensor's accuracy?

[Ron Alan]

First off...13.2 at 5500 would not concern me if the rest of the AFR followed a normal downward trend...so at rev-limiter you are maybe 12.5? 60 psi is about all a stock fuel pump puts out...and at that with a restrictor the cars run very fat. I'm with Chris...ECU pre FP regulator rule change!

[DrDomm]

Have you tried a different ECU?

 

Last weekend, I changed the ECU.  I really wish I had done it during the season...but it's such a pain, and I didn't quite understand how it might be a cause.  I went from the '99 BP4W to the '00 BP5R.

 

I'm with rob on this one. If the original car was purchased used as a race car there is a strong chance it has a modified Ecu. Old school modified ecu's required 58-62 psi as that is what the factory regulator produced.Another issue would be that the Ecu also has timing advance built into it and if you are setting the timing to 15 base you will also be getting detonation that is costing power and possible valve damage

 

Hmmm...but I'm pretty confident the one that was in previously was untampered.  It was the one in my '99...that didn't have this problem.

 

Have you verified the pressure sensor's accuracy?

 

Fuel pressure? Yes...sort of.  The regulator has a gauge on it, and I have a fuel test port on the "return line" in the trunk.  I attach a fuel pressure gauge to the test port.  The gauge on the regulator does read about 3-4 psi below what my data sensor logs, but the gauge on the test port is only off about 1-1.5 psi...lower.  So, the gauge on the regulator will read about 56, the gauge on the test port about 60, and then 61 from the data (read at the front of the car).

[DrDomm]

First off...13.2 at 5500 would not concern me if the rest of the AFR followed a normal downward trend...so at rev-limiter you are maybe 12.5? 60 psi is about all a stock fuel pump puts out...and at that with a restrictor the cars run very fat. I'm with Chris...ECU pre FP regulator rule change!

 

Yeah, around 12.3-12.5 at redline.  The thing that makes me really wonder though, is the fuel mileage.  I just wonder if torque is suffering (need an excuse   )

 

I hope you're correct about the ECU.  I'm gonna look and make sure it was the one from my '99, and not the spare I had laying around (from eBay).  I was swapping a bunch of stuff last spring before I knew the timing was off.

 

Anyone have thoughts on sensors?  Crank, Camshaft, MAF, air temp?  I had swapped the first 3 before figuring out the timing issue.  EGR?

 

I'm just looking for ideas/experiences.  It'll be a while before I have a chance to swap out some of these things, and get it to a dyno to check.

 

Thanks everyone!

[chris haldeman]

If the egr has failed open that could mess with the fuel requirements and power. Also look for a leak upstream of the wide band screwing with the numbers tho if you are tuning for max power and not just setting fuel curve I doubt it's the issue

[Ron Alan]

12.2 at the top is about what mine is with 58-59psi...to me this is nice and fat?(others?)...usually your best torque numbers but a little off on the top end. Check your plugs as well for any signs of a lean condition...but my guess is you are not lean but just think you are. Pressure gauges vary so trust your wideband/dyno wideband(these should be very close) and dont wory what the FP reads. Just keep track of what works. If you lower the FP and get the 13.2 number(lowest it goes) at red line then you are pushing the lean envelope.

[Steve Scheifler]

I may have missed it but where is the wideband sensor and have you checked it for accuracy? When feeding the dyno from a tailpipe mounted sensor we generally see about 0.4 AFR leaner than at the collector or downpipe even without a conspicuous exhaust leak along the way.

So far a modified or otherwise unusual ECU sounds like the best guess.

[DrDomm]

I may have missed it but where is the wideband sensor and have you checked it for accuracy? When feeding the dyno from a tailpipe mounted sensor we generally see about 0.4 AFR leaner than at the collector or downpipe even without a conspicuous exhaust leak along the way.

So far a modified or otherwise unusual ECU sounds like the best guess.

 

Wideband is in the downpipe.  It's read very similar to tailpipe sensors on different dynos.  My bung in the downpipe does have a small leak that I'm re-welding.

 

I also did find a small exhaust leak from the manifold at the 4th cylinder.  Tightened down the nuts, and was better.

 

Even if those leaks were giving false numbers (and I don't know how much they affect it), I'm still curious about the 12.3mpg.  Does anyone get that in a '99-'00? 

 

If I get a chance, I'll try to post photos of the plugs...one was more tan that the others...but this was after warming the car up for a leak down test.  I'm not sure how accurate that is.

 

Again, thanks everyone for the ideas!

[Bench Racer]

Yeah, around 12.3-12.5 at redline.  The thing that makes me really wonder though, is the fuel mileage.  I just wonder if torque is suffering (need an excuse   )

Did you some time back post a AFR chart? If you have a chart post it again. Is 12.5 at redline normal for a 2002?

[DrDomm]

Did you some time back post a AFR chart? If you have a chart post it again. Is 12.5 at redline normal for a 2002?

 

This is from prior to the valve job, but I had it as a computer file.  This is a 2000, not 2002.

 

 

Gonna be away for about a week, so sorry if I miss any replies.

[Danny Steyn]

Something seriously wrong with that AF trace on a 99/00 Miata!

[Tom Sager]

Domm, there are so many possibilities here and at the risk of sending you in more directions here are a couple thoughts.

 

You could have 1 injector that is messed up which is causing an aggregate lean reading.  Look at plugs as suggested for one that looks different.  Have you changed the airflow meter?

 

I once had a similar situation and the throttle positioning was the cause. 

[Jim Drago]

Something seriously wrong with that AF trace on a 99/00 Miata!

Domm

Not sure what Danny is referring to.. But That a:f in the graph(especially red- green) looks like every 99-00 we have ever done here.

 

Here is what we typically see around 50-52 psi on 99-00 Sm above and below 180 degree water temps. Mine is a dynojet and Bret uses something different, the shape varies a little dyno to dyno and where sensor is located.

Jim

 

 

[Tom Sager]

Oops I didn't see the part about rebuilt injectors having been installed.

 

Shape of curve does look fine but at a true 60 PSI (or even upper 50's) it should be way richer than the turquoise and orange lines show.  Don't you think so Jim?

[Jim Drago]

Oops I didn't see the part about rebuilt injectors having been installed.

 

Shape of curve does look fine but at a true 60 PSI (or even upper 50's) it should be way richer than the turquoise and orange lines show.  Don't you think so Jim?

I do, but have had one that I changed anything and everything people could think of on here and it ended at 57psi, which is 6-7 lbs more than EVERY other 99/00 I have built. Ran great, made great power, but always needed more psi than all other cars in same conditions. It bugged the hell out of me and at least two or three full day attempts at rectifying this "problem", finally gave up as the "problem" was only a problem in my mind, it didnt act any differently on track and was no symptom of other pending pending.. Nonetheless it did bug the hell out of me

[Ron Alan]

^^^^ Similar Miata (SM) 

There is "normal"(which is a range)and sometimes a little outside the norm...kind of like dyno numbers across the country, you will drive yourself crazy trying to repeat/get what someone else has published when in reality your numbers are fine! 

Jim's AFR is what I see...not sure why you think 13.2 is too lean at 5500 rpm Domm?

[Bench Racer]

When checking plugs, pay pictular attention to #4.