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Old 11-19-2017, 04:05 PM   #1
hypnonotic
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Default RPM stuck at 4-5k on WOT and hard warm starting

I have an interesting issue with my '13 WRX with no CEL.

The vehicle will not start when warm (fine when hot, and fine when cold.) Cold starts are fine and strong (even in the coldest day so far of 20F), restarting the car within ~5min of shutdown results in a strong start. Waiting 10-15min after shutdown will result in cranking but no firing and I will have to wait an additional 20min for it to cool down completely before it will start again. I narrowed it down to coolant temps roughly between 160-110. I don't have an oil temp sensor so I can't provide that info.

The shop that had it for a week said everything on the computer checked out and it had fuel pressure on cranking, but they were only able to recreate it once. (I don't think they actually got it up to temp to be honest) so I'm not sure if I trust what they said about the fuel pressure/sensor info. Also there is still dust and dirt on the fuel lines where they would have had to put in the T-fitting, it doesn't look like it was touched.

The problem started when the weather got cold out (Temps are now 20-40 and problem happens at all temps and also in a 70 parking garage), but it continued to happen on a freak day at 55 degrees, so it's probably unrelated.

Things I tried:
1. Starting in warmer air (parking garage) -> resulted in same starting issue, it made the problem worse since it took longer to cool down.
2. Pulling battery to reset fuel trims.

My current guesses:
1. Bad Fuel pump
2. Bad IAT
3. Dirty MAF (probably not)
4. RT6 heavier oil (probably not since cold starts are fine)

A bit more information on the car:
I bought is a few months ago used with 40k miles. Has PW top mount IC, cobb downpipe and exhaust all the way back. GFB BOV and a perrin intake. Sadly I bought it from a Ford dealership (pretty cheap since it had been on their lot for awhile) so I didn't even get a full list of mods and I have discovered more things as I work on the car. I listed everything engine related that I know of.

I did have a different issue when I first bought the car (Probably why I picked it up for 3k under market value)

My main problem is that I live in a smallish town and the only tuning shop is 1.5hrs away and booked out for the next month or two and my local shops have had no luck with diagnosing the problem (same story with the engine harness issue, luckily I found a thread on this forum with the fix).

Any other info I might have missed, I'll be happy to provide. Any thoughts or comments to guide my troubleshooting would be gladly appreciated!

Thanks!
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Last edited by hypnonotic; 12-10-2017 at 11:59 AM.
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Old 12-10-2017, 12:03 PM   #2
hypnonotic
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An update so maybe someone can throw an idea my way:

The WOT problem that I originally mentioned doesn't seem to be a problem, idk what it was but it's not happening now.

I got a fuel pressure testing valve put on this weekend:


Looking at 80-90psi on the fuel rail. Not sure if the car is tuned for this, or if I have a bad FPR. Too much is unknown about this car.

Anyone have any thoughts on fuel pressure being too high and causing this issue?

Last edited by hypnonotic; 12-12-2017 at 05:35 PM.
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Old 12-11-2017, 07:32 PM   #3
hypnonotic
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Ok, so a quick update.

Tonight I recreated the no start condition. While it was in that state, I attached my pressure gauge. Unfortunately, it depressurized while I screwed in the schrader valve, however after cycling the ignition it started right up without a problem.

So this seems to me like it might have fuel boiling in the fuel rails after sitting for a bit, though that should be fixed by cycling the ignition a few times and flooding out any hot fuel. I've tried priming about 10 times, and it doesn't help.

Any other ideas on what the problem might be?

Last edited by hypnonotic; 12-12-2017 at 09:54 AM.
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Old 12-12-2017, 02:26 PM   #4
hypnonotic
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Finally had a hit with the search tool:
https://forums.nasioc.com/forums/sho....php?t=2797642

I'll have to look for a leaky FPR.

Does anyone know if the stock FPR would be more likely to fail under a constant higher fuel pressure? It seems like the car is tuned for this higher pressure from the previous owner since it runs very well once started.
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Old 12-12-2017, 03:00 PM   #5
thewolfe
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80-90 psi is a lot of pressure when stock is near 40. When I first installed my FPR I accidentally set it to around 60 which made my id1000 injectors quite noisy and the car ran like pooh. Obviously because it wasn't tuned for that high pressure. Set it the correct pressure and everything went back to normal. If you're on the stock regulator and you're seeing pressure that high there is a problem.
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Old 12-12-2017, 03:49 PM   #6
hypnonotic
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Quote:
Originally Posted by thewolfe View Post
80-90 psi is a lot of pressure when stock is near 40. When I first installed my FPR I accidentally set it to around 60 which made my id1000 injectors quite noisy and the car ran like pooh. Obviously because it wasn't tuned for that high pressure. Set it the correct pressure and everything went back to normal. If you're on the stock regulator and you're seeing pressure that high there is a problem.
I'd like to reduce the pressure, unfortunately, I think the previous owner may have it tuned for that high of pressure since it seems to run great, and AFR looks ok at WOT (11-12:1).

Would the ECU be able to account for that much differential in pressure without a tune?

I'm hesitant to turn down the pressure if the car is tuned for it. Though the extra pressure might explain the WOT throttle issue that I used to have (which seems to have cured itself) where it would bog down just after getting into boost. Would a rich condition cause hesitation during WOT, and would the ECU be able to learn to fix that? I was under the impression that WOT didn't log AFR and didn't try to correct for it.

I'm going to do two things tonight:
1. Run it with the pressure gauge attached and see if the pressure has bled off when I go to start it next.
2. Check the vacuum line for fuel.

Is it safe to drive with a pressure gauge attached to the schrader valve?


I went ahead and highlighted my questions.

Thanks for any help again!

Edit:
Quick update that didn't warrant a new post. FPR is definately stock. It's not visibly damaged either. I didn't take the vacuum hose off yet, but will report back after that.

Last edited by hypnonotic; 12-12-2017 at 06:42 PM.
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Old 12-12-2017, 07:22 PM   #7
viper_crazy
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Re: leaving fuel pressure gauge on while driving. As long as its not interfering with any critical components, I don't see why you can't leave it on. They make fuel pressure gauges that mount in your car so you can monitor it all the time, so leave it on, I say.
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Old 12-12-2017, 10:51 PM   #8
hypnonotic
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Quote:
Originally Posted by viper_crazy View Post
Re: leaving fuel pressure gauge on while driving. As long as its not interfering with any critical components, I don't see why you can't leave it on. They make fuel pressure gauges that mount in your car so you can monitor it all the time, so leave it on, I say.
Hi! Thanks for the response.

The main reason I was worried is because the gauge maxes out at 100psi and if I'm pushing 90psi at idle I didn't want to blow the gauge up if the it increased under higher RPM.

As for what I did tonight, I went for a short drive where I specifically didn't get the temp up because I was planning to drive shortly after. Interestingly enough, I got the same no start condition, so I'm no longer convinced it has any relation to engine temp despite previous correlation. I also replicated blowing off the pressure and getting the car to start.

I closely examined the fuel system and I read through the factory service manual. I can now say 100% that I have a stock FPR and a stock fuel pump. The FPR does not have any signs of tampering or damage, so unless there is a covert way of deliberately raising the fuel pressure, without adding an adjustable FPR, I'm not convinced the previous owner did this.

Which brings me to the WOT problem that I mentioned and then dismissed. I did a hard pull on the entrance ramp tonight as well, and sure enough that strange pause around 4-5K rpm is still there. I think I have been unconsciously not fully putting my foot down, and with the cold weather, the car isn't warmed up fully by the time I make to an entrance ramp and I deliberately don't pound on it when it's cold. I think this lead me to believe the problem was gone.

That being said, I'm no longer convinced that the previous owner did this deliberately and tuned for it. Additionally from what I've read about adjusting fuel pressure here https://www.aeromotiveinc.com/advanc...fuel-pressure/ (see below for the specific quote) leads me to believe that it would still be drivable since the ECU is correcting for normal driving throttle positions.

The one last thing that I need to do before I can confirm this to remove the vacuum line on the regulator. Not only do I need to check for fuel in the line, but from some other sources, that is how you measure the base pressure setting of the regulator.

Thanks everyone for the help! If you see any problems with what I just said, let me know. I don't want do anything stupid lol.

"Adjusting the base fuel pressure of most modern, EFI engines will initially affect both the WOT and idle/cruise AFR. However, the permanent affects are mainly to WOT AFR only. Make fuel pressure changes based on WOT AFR and ignore drivability/ cruise AFR until WOT is correct and safe!

During low load cruise, most engine management computers run in “closed loop”, using the stock O2 feedback to constantly trim injector pulse width (fuel delivery) towards the lean best AFR of 14.7:1. This target A/F is commonly referred to as “stoich” or “stoichiometric” and is the chemically correct balance of air and fuel for a complete burn. The computer “learns” what is necessary to maintain “stoich” and stores this information until the learned memory is cleared by disconnecting the computer from the battery for 5 minutes or more."
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