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Discussion Starter #1
Been trying to diagnos my car for the past few months. It kind of sounds like I'm hitting the rev limiter cuz the car pops, backfires, or sputters. It sounds like my rev limiter but it can't be cuz it happens at 3 and 4 grand sometimes 5 grand, and sometimes not at all. I can ease past it by letting off a little bit, still accelerating just not as hard and the rpm's will climb on up to 6. My rev limiter is set at 6500, stock setting is 6250. I did a compression test all the holes tested 175 except number 3 which was 145. I've also tested my coil packs by ohming them, I've got a junk tester they all come back .001 whatever that means but their all the same. I'm running stock lifetime warranty coil packs and Seimens Deka 60 lb injectors EV1. Does anyone know the minimum requirement of rail pressure for these injectors to function properly?:facepalm:

---------------------------------- Value------------Min------------Max------------Avg
Pressure Drop Fuel Inj--------------40.67------------29.96----------45.24----------40.04
Fuel rail pr psi----------------------40.38------------29.27----------45.91----------40.21
FP duty cycle----------------------0.26--------------0.13-----------0.30-----------0.26
 

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Head Unicorn
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What you're describing sounds like fuel starvation to me. It'd be nice to datalogyour car and capture the moment this is happening. Even though you may be be getting any DTCs, scan your car's EEC and if you don't have that ability have a shop scan for PENDING CODES. Pending codes are problems that haven't been verified yet. My suspicion is that you'll find a fuel related code. The easiest guess is a fuel pump failing, but that's not necessarily the problem. This could get very expensive at your 12 psi of boost with a fuel starvation issue.
 

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US Air Force (retired)
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It you are data logging then the fuel pressure should be a constant 39-40 psi across the fuel injectors. If you are testing fuel pressure at the fuel rails then its 39 psi minus vacuum/2 or 39 psi plus boost. For example my car idles at 18 in.hg. 39 - (18/2) = 30 psi. At 6,000 rpms when I'm running 9 psi of boost, its 39 + 9 = 48 psi. The computer also increases the fuel pressure at the rails during hot days to prevent vapor lock. I have seen my fuel pressure gauge read as high as 58 psi at idle. I have had my car data logged when it was doing this and the data log said 40 psi.

Any more than that is over my head. CliffyK knows the technical stuff.
 

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Head Unicorn
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The computer also increases the fuel pressure at the rails during hot days to prevent vapor lock. I have seen my fuel pressure gauge read as high as 58 psi at idle. I have had my car data logged when it was doing this and the data log said 40 psi.

Any more than that is over my head. CliffyK knows the technical stuff.
I found this issue to be problematic with nitrous and couldn't figure out why it was happening. After many datalogs I finally found the two parameters that caused the increase: ambient air readings of 100+ degrees and ECTs of 200+ degrees...... don't really know why yours wouldn't show up on datalogs... mine datalogged as high as 65 psi.
 

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Discussion Starter #5
how can I post a datalog?
 

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Head Unicorn
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---------------------------------- Value------------Min------------Max------------Avg
Pressure Drop Fuel Inj--------------40.67------------29.96----------45.24----------40.04
Fuel rail pr psi----------------------40.38------------29.27----------45.91----------40.21
FP duty cycle----------------------0.26--------------0.13-----------0.30-----------0.26
Guess you added this after my first post. This doesn't do much for me. Mins and Maxs don't tell much of a story. The fuel pressure will see a huge drop on shifts, especially under boost so a minimum value doesn't confirm a pressure problem. You need a full datalog or being that I see you're self tuned with PRP why don't you watch it in real time and see what the cause is. It's going to be a fueling issue so you need to watch FRPS, FPDM, duty cycle and the one thing you can't watch... the fuel pump.
 

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Head Unicorn
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I failed to mention.. and it should go without saying: the datalog needs to be the data during the problems, not data from a no problem run.
 

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US Air Force (retired)
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I found this issue to be problematic with nitrous and couldn't figure out why it was happening. After many datalogs I finally found the two parameters that caused the increase: ambient air readings of 100+ degrees and ECTs of 200+ degrees...... don't really know why yours wouldn't show up on datalogs... mine datalogged as high as 65 psi.
Ahh, I learn something new everyday. I don't have the ability to datalog. That was what I was told by the dealership that did the datalog. Obviously, they did their datalog with the engine cool.

About 5 1/2 years ago when I had the supercharger and gauges installed. I had never seen an aftermarket fuel pressure gauge on a returnless system before. And, I had not yet joined Modded Mustangs. It took a while for me to learn what was going on. When the fuel pressure hit 58 psi at idle, I changed the FRPS thinking it was bad. (I kept trying to buy a regulator but our cars don't have them.) :dunno When it did it again, I took it to a dealer to trouble shoot the problem. They found nothing wrong but charged me $200. Oh well, live and learn. :)
 

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Head Unicorn
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Stealerships love to charge you money for not fixing things and for giving no explanation why they didn't fix anything. Occasionally they'll fix some **** that's not broken and bill you for that too.
 

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Discussion Starter #11
Yes the datalog does show a consistant 38-39lbs during moderate acceleration at 60 some mph and my low points, yes, are during gear shifts. At first I was thinking fuel starvation cuz I can control the sputter with the foot feed during heavy acceleration. I can make it sputter back out and make it sputter then back out on wheel, but at those times I'm showing good enough fuel rail pressure.
 

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Pressure does not equate to volume. You can have enough pressure, but still undergo fuel starvation.
 

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Discussion Starter #14
I re-tweeked my tune. Well actually I wrote a brand new one from scratch. Kept it rich at idle and ran it fat all the way through. WOT showing a green 11. Had good pull all the way through without the sputters and popping. I also pulled 4 degrees out of timing. Which I plan to tune back in as I lean it down. That's that in a nutshell.
 

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Head Unicorn
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I don't know what a green 11 is, but I'm going to assume that its AFR. Glad you've more or less solved the problem.
 

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Pressure does not equate to volume. You can have enough pressure, but still undergo fuel starvation.
While that in theory is correct, if the cylinders suck more fuel out of the fuel rails than can be replaced(flow) then pressure will go down.

Pressure is the driving force, flow does the work. Kind of like electricity. Voltage is the driving force, amperage does the work.
 
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