Fuel pressures and AFRs all over the place

Hi all

I've been having some trouble with the AFRs on my car. The AEM AFR gauge was all over the place, sometimes rich off the scale, and sometimes lean off the scale. I was advised that I might have a faulty wideband O2 sensor, so I replaced it.

Ryan then kindly remapped the car for me, and confirmed that my O2 sensor was reading accurately. Sure enough, the AFRs were reading very different from when the car had previously been mapped. Ryan and Darryl did notice that the Aeromotive FPR was a long way off what it had previously been set to.

When we left it, the FPR was reading 25 on idle and the AFRs were fine.

A few days later, the AFRs started reading really rich again, about 10 on idle. I checked the FPR and it still seemed to be on 25, so I didn't think much of it. But I checked the FPR again today (after having driven the car for 15 minutes) and it was reading 40 on idle!! So I adjusted it back down to 25, and sure enough, the AFRs looked fairly healthy again.

Then I drove the car another 10 minutes and then checked the FPR again. This time it was reading 20! So I raised it back up to 25.

So, in a nutshell, I think my AFRs are messed up because the fuel pressure wont hold. Does anyone know why my fuel pressures keep jumping around?

Is it likely to be a dodgy FPR, or could it be something else?

Thanks.

*** Summary below, for those who dont want to read every post! ***

The fuel pump, fuel filter, Aeromotive FPR, gauge, return hose in the engine bay and all vacuum hoses have now been replaced. (In fact, I've now tried three pumps and three FPRs.) I've also replaced the IACV and the fuel pulsation dampener has been bypassed.

The problem remains and appears to be this:

When driving for a while, the fuel pressure on the Aeromotive drops 10-15 psi, according to the gauge in the engine bay. At the same time, the Aeromotive gets very hot. I'm pressuming this could be the cause, but I guess it could be an effect. Since I've now tried three Aeromotives, I'm convinced they're working fine. That said, it's possible they're getting hotter than they should.

Fuel pump wiring and voltage supply has been checked by Matt Harwood and remains constant. I've even been switched to 12V permanent, but that hasn't helped either.

*** Another update!

I've now got a fuel pressure gauge in the cabin, which reads rock solid fuel pressures. So the drop in pressure in the engine bay must be because it was getting hot. Mark at Phoenix assures me the car has no fuel pressure issues at all.

I also bought an oil temperature gauge, and Mark has confirmed that the car goes lean when the oil temperature exceeds 70 degrees. So that's what I need to look into next!

sounds like the FRPR, the pump, or the power supply to the pump...? they can be the only 3 things... right?

Going to be a faulty regulator if you ask me mate, thats where i would start anyway. Have you got anyone you can borrow one off of for a few days to see if it fixes it?

Thanks guys. I thought it might be the regulator.

I don't know anyone I can borrow one from, so it looks like I'm gonna have to buy a new one. More credit card stacking for me...

Sounds likely, could have a split in the diaphragm, i had a cheapo FPR on my Supra and that made the AFRs go all over the place.

Just to keep things interested, I've been checking the static fuel pressure on my FPR. Each day I check/set it to 40psi. But one day it was reading 35, so I upped it to 40. The next time I got out the car it had dropped to 32, so I upped it again. Sometimes it seems to fluctuate the other way!

I was about to buy a replacement FPR, but then I was advised that the Aeromotives never fail and it was more likely to be my fuel pump or the fuel filter (which is due anyway). So I've ordered those two bits, given that they're the cheaper bits to replace anyway. Fingers crossed that these do the trick.

Bloody Supras.

Daft question I know, but I presume you've done up the nut on the adjuster nice and tight to make sure it can't vibrate around and change the pressure?

I was about to buy a replacement FPR, but then I was advised that the Aeromotives never fail and it was more likely to be my fuel pump or the fuel filter (which is due anyway). So I've ordered those two bits, given that they're the cheaper bits to replace anyway. Fingers crossed that these do the trick.

 

Bloody Supras.

Who told you that Darren?

Never say Never!

I did do up the nut nice and tight, but good thinking anyway.

It will be the regulator - nothing else.

Where is it - is it worse when hot? Does it always read the same (let's say) 25psi from a cold start?

Everything points to it being the FPR, but I've not heard of a faulty Aeromotive before (though as Ryan says, never say never)

The filter is due to be changed on the next service so no harm in doing that now. The pump is a possibilty, but the walbro's are cheap. Besides if it's not the pump Darren can have a twin pump setup

The what now? I'm liking the sound of it already.

The what now? I'm liking the sound of it already.

Thought you might I'll explain next time we speak

Im still sticking with it being the FPR

Could be pump, pump filter blocked, fuel filter blocked or fpr. We've had plenty of Walbro's go weak or fail on us causing this exact problem and personally never had an Aeromotive go down as yet, so pump has got to be the place to start for me. Hope it works! Might have a 2nd hand Aeromotive coming up for grabs in the next couple of days if you need one though!

Yea i have never heard of a faulty Aeromotive FPR.

Also how are you checking the fuel pressure, is the vacuum line connected or disconnected when you read the gauge? if its connected, you could have a vacuum leak elsewhere in the engine bay causing it to be off and thus inconsistant. It would be easy to start here but i'm also in aggreance with Paul, check the fuel filters in both the tank and the fuel line.

Check the wiring to the pump as well.

Yea i have never heard of a faulty Aeromotive FPR.

 

Also how are you checking the fuel pressure, is the vacuum line connected or disconnected when you read the gauge? if its connected, you could have a vacuum leak elsewhere in the engine bay causing it to be off and thus inconsistant. It would be easy to start here but i'm also in aggreance with Paul, check the fuel filters in both the tank and the fuel line.

Darren isn't online, but I previously owned this car and have been trying to help with the new issues.

The pressure is checked with the vacuum line connected, the diag ports bridged and ign on 2 (i.e just the pump running). From what Darren's said the pressure fluctuates quite randomly.

Good point on checking the vac hoses, however the line was replaced with a new one just one week ago (by a reputable garage - Pheonix). It's not tee'd at all and goes direct to the stock mounting point.

I'm certain it's one of the 3 things mentioned. The pump is about 3 years old, the fuel filter and fpr about 2.5 years old.

For it to be the pump or filter then it would need to have been blocked/faulty during the mapping/setup and whole period of healthy running - and the fpr setup to give 25psi under a “low” fuel flow rate.

So the fault then temporarily rectifies itself and the AFR's go rich as the flow rate bumps way up closer to a normal walbro flow - and some how the FPR cannot maintain pressure control under this increased flowrate (????).......I find all this a lot harder to believe than just a faulty fpr (or related fault - vacuum/pressure feed etc).

If I had to bet....... I'd be betting on the FPR (or something related to its installation).

We'll soon find out

For it to be the pump or filter then it would need to have been blocked/faulty during the mapping/setup and whole period of healthy running - and the fpr setup to give 25psi under a “low” fuel flow rate.

 

So the fault then temporarily rectifies itself and the AFR's go rich as the flow rate bumps way up closer to a normal walbro flow - and some how the FPR cannot maintain pressure control under this increased flowrate (????).......I find all this a lot harder to believe than just a faulty fpr (or related fault - vacuum/pressure feed etc).

 

If I had to bet....... I'd be betting on the FPR (or something related to its installation).

The only thing thats recently changed was relocating the FPR, however there's two considerations:

1. It was done by Mark at Pheonix, so a bad installation is extremely unlikely

2. The problem *may* have existed previous to this, but was unable to determine AFR's at due to a b*ggered wideband O2 sensor (since replaced)

I agree with your logic though

We'll soon find out

Indeed

It would be interesting to note the vacuum on a boost/vacuum gauge when the FP is at 35psi, 25psi, and 40psi. Like is the vacuum on the gauge the same the entire time? Is the idle RPM the same as well?

Do you mean adjust the fuel pressure with the engine running and compare to the manifold vacuum? Or do you mean tee'ing a gauge into the line between the manifold and the feed to the FPR?

I must admit - I always regarded the Aeromotive units as reliable and well proven and have used them before with no issues at all. It would be the first one I have heard of failing but from the symptoms described so far it seems to be a potential culprit to me.

Could be the fuel pump speed control computer having a spazz unless you have run a bypass or have the 12v mod.

Lyndon.