Running rich, faulty fuel pressure regulator?

Technical Area Engine, Transmission, Exhaust etc
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  1. My model of MX-5 is: __1996 Mk1 1.8
  2. I’m based near: __
  3. I’m looking for technical help or recommendations on: __ Diagnosing possible cause of engine running rich

My Roadster has struggled through emissions for a few years, and munched catalytic converters.

The engine is on about 120k kms, but the ancilliaries (original engine) are more like 170k miles on them (original inlet manifold and bits were fitted to the replacement engine fitted many many years ago).

The garage that looks after the car has tried alternative Mass Air Sensor, which “worked” for a while. Tried new water thermisters. I’ve got a new O2 sensor on… New(ish) spark plugs. Last time, for the MOT it was sent through with a loaner cat, and then the old one refitted

At the next MOT the garage is suggesting looking at doing a top end rebuild.

While idly researching, I thought about perhaps a failing fuel pressure regulator.

While the engine pulls strongly (it doesn’t feel worn out), fuel consumption seems ok (though it doesn’t get driven much these days, enough for me to closely monitor consumption), there are a number of symptoms:

  1. Sooty tailpipes. Though all the MX5s I;ve ever had has had sooty pipe. But no visible smoke.
  2. Idle when hot; when pulling up, the tick over will dip down, and the engine will bog down. I’ve become a bit of a blipper of the throttle.
  3. Starting; it cranks fairly slowly, but always starts. New battery. Sound terminals. Replaced the earth/ground from battery to PPF.

I;ve yet to check the vacumn line on the FPR; my understanding is if I smell fuel, the FPR is shot. I was hoping to find a replacement part fairly cheap, and just swap it in. But rather shocked that I can’t find a UK supplier, only in Germany, and the Mazda part is fairly expensive. No pattern parts seem to be available in the UK it seems (unless someone can point me in the direction).

The other possibiklity is a hairline crack in the MX5parts 4-2-1 header that I fitted back in 2005. A few years afterwards, one of the welds on a header started to fall apart, but I had the manifold welded by an engineering company. The repair still looks perfect.

Any thoughts on my thinking? If engine rebuild is needed, then that might be me done with this car, and call it a day with MX5s. Obviously I am trying to avoid a 4-figure bill on a car with 270k kms on it.

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I’d think a crack in the manifold is likely, especially as similar has happened before. I presume you’ve checked for stored fault codes but as you’ve changed both the coolant temp sensor and the lambda sensor they’re likely okay.

Failing FPRs aren’t something one often reads about, so they’re probably reliable and I’d have no qualms about buying a used one from a breaker.

The bogging down when coming to a stop probably just needs the IAC valve cleaning and base idle resetting.

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Set your base idle mixture, are you running any aftermarket add on’s, cone filter etc?Are the injectors ok, and not over fueling in a wrong pattern, i hit this with one, i’ll dig out some pics…

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M-m

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The car has ran aftermarket filters since it was built; imported by Mazda UK in 2000 as a 4-year old car with HKS intake. Now newish Ramair. For a while I had a Fast Eddie cotton filter. 4-2-1 header since 2005. Arc cross over pipe (checked for damage) since about 2007.

Looking back, high CO failures started in 2016, 4000kms ago (the car is not used much anymore). New cat then. Failed the next year, cat replaced under warranty. Passed 2018. Fail again 2019. Failed again, 800kms later, in 2020, during COVID times. Shock pass in 2021. Failed 2022. I’m expecting another fail. Opportunities to tinker are limited these days due to other duties.

Garage (MX5 racing team) say the injectors are not leaking and are fine.

They’ve set the base idle, I’ve set the base idle.

A 27 year old, 270,000 km FPR has not been checked. There are 4 parts to a FPR, but the main ones that can fail are a spring and the diaphragm itself. I suppose external faults can develop over a 27 year period, such as the FPR vacuum line.

Unlike last time it cracked (which was about 2007-08, well before these failures), the manifold has no visible cracks (the crack then was visible and sooty, but the car still passed even like that, which was a long time ago). I have not had it X-rayed (nor would that be worth it). Not gas tested it with something like Seafoam.

1 Like
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She is always going to have issues with whats been added, I hit the same problem , the biggest issue is the MOT test on how they are changing over the years,unfortunately even if you try and push your 96 as needing to be tested as an import and not an MX5 , they will say no because of the year of her being built .
The injector i posted up was not leaking, but she was running rich ( G Ltd) getting deep found the damaged injector, i was shocked to find it but cured the issue.
I need to go as my break is over , but there is a lot with all this to get involved in.
M-m

6

No issues like this in the first 11 years of ownership. Maybe the car is just old.

Car was first used (Japan) after 1st August 1995 and so needs a cat test. Always been the case AFAIK.

Had the G-Limited had injector work before. How did it break if it had been there from the factory? Looks like someone bashed it.

The only person who would have previously touched the injectors in my car would have been Andrew Stott. I can ask to check in the Fall (which is complicated as I’m out of the control trying to get married) the injectors again. Wary of pulling them myself in case I am left with a car peeing petrol everywhere after.

I can still check the FPR. Its a quick check for a split diaphragm. With an old car like this (old in terms of miles), I can’t rule anything out. Most of these Mk1s don’t make 120k miles, either being scrapped or becoming occasional sunday drivers (old cars now). I reached 270k kms 7 years ago, and since then, averaged less than 1000kms a year. In 2000, people on the predecessor to this forum used say the MX5 was really good for rust compared to Fords etc. Look at what happened since.

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Fuel Pressure Regulator is very easy to change; why haven’t you just done that?

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Like you state, its an old car and process of elimination, as are most of the faults these days with no fault codes we hit regally now.
The G Ltd had no work carried out to her on that side of things, we can tell if there as been any work on these roadster as we look and feel in the removal of anything, but it was nice to find and cure the issue on her, as i guess you know we have many many strange story findings on searching for faults and putting them right over the years.
M-m

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Exactly. But now this issue has got to be parked due to health reasons, for a while.