Fixed knock issue with new MAF, now I have a fuel issue?

krisking94

Member
Joined
Oct 6, 2014
So I replaced my MAF and it immediately got rid of my knock issue. Did some WOT pulls after I drove it around a bit, did really good no knock what so ever. Was the first time in probably a year since the car has hit full throttle like that. After that I decided to lower my FP because I had increased it a bunch before to see if that would help with the knock. Before it was at like 48 line on and it jumped to almost 60 line off. I changed it to 43 line off and it dropped to about 34 line on. After that did did a couple more runs, got some knock. Changed it back up to what it was at. No knock. Started car this morning, started just fine. I let it run for about a minute then it stalled out. Started it again ran for a couple seconds then stalled out. Well thought it might have been the fuel filter because my brother who previously owned it said it had been 5 years since it had been replaced ( I know) Got it in car started up. Set FP back to 43 line off. Drove it around went fine, got it back home then it stalled out before I put it in park. Put some fuel cleaner in it, and bumped the fuel pressure back up to what it was at previously. Did another pull, eased into the gas then went foot to the floor. No knock except shifting thru the gears (came up, then immediately went away after gear change) parked it, then I went for another drive and it started to sputter again like it was starving for fuel. Went back home, fuel pressure at rail was at about 45 line on. Weird thing is I couldn't hear the fuel pump prime up from inside the cabin like normal. If you got real close to the gas tank then turned the key you could hear the prime pump but hardly. At some points I felt like I couldn't hear it prime at all. I think what's going on is the pump and filter both went bad after those first couple of hard pulls, hadn't been pushed like that in some time. It seemed like every time I would check the FP it was inconsistent with what I had last set it at. So my brings you to my question, is the fuel pump bad and only showing sings intermittently? I have a fuel pump at home already for the car.
 
On my t type and on this GN I could always hear the pump prime while sitting in the car doors closed. You could always distinctly hear it.
 
Mine depended how much fuel was in tank. Full tank I wouldnt hear pump prime.

Time to rent a fuel pressure gauge for a few days to see if its pump or regulator.I get mine from Autozone.free to rent
Watch the pressure with that, to be positive.
 
After changing the MAF and fuel pressure you might want to start by reseting the computer by disconnecting the power for a minute. Not sure who's chip you're using but if it learned your fuel curve at 60 PSI and now it's 43 that will make a difference. I would also start by checking TPS and IAC numbers after ECM is reset. You may have a fuel pump issue but I would start with the ECM. Do you have a fuel pressure gauge you can see while driving or are you logging Fuel pressure?


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After changing the MAF and fuel pressure you might want to start by reseting the computer by disconnecting the power for a minute. Not sure who's chip you're using but if it learned your fuel curve at 60 PSI and now it's 43 that will make a difference. I would also start by checking TPS and IAC numbers after ECM is reset. You may have a fuel pump issue but I would start with the ECM. Do you have a fuel pressure gauge you can see while driving or are you logging Fuel pressure?


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I just have a gauge at the rail. I am using a TT alky chip.
 
After changing the MAF and fuel pressure you might want to start by reseting the computer by disconnecting the power for a minute. Not sure who's chip you're using but if it learned your fuel curve at 60 PSI and now it's 43 that will make a difference. I would also start by checking TPS and IAC numbers after ECM is reset. You may have a fuel pump issue but I would start with the ECM. Do you have a fuel pressure gauge you can see while driving or are you logging Fuel pressure?


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Tps is at .44 at idle. Iac bounced around from 30-36
 
After changing the MAF and fuel pressure you might want to start by reseting the computer by disconnecting the power for a minute. Not sure who's chip you're using but if it learned your fuel curve at 60 PSI and now it's 43 that will make a difference. I would also start by checking TPS and IAC numbers after ECM is reset. You may have a fuel pump issue but I would start with the ECM. Do you have a fuel pressure gauge you can see while driving or are you logging Fuel pressure? Now before resetting Ecm should I set FP back to 43-45 line off?


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Yes I have thought resetting the ecm, but then I would have to let the car re learn everything. I'm afraid while doing the 15 minute drive to re learn the parameters like Eric states in the instructions the car will leave me
 
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Those rail gauges are known to be inaccurate so you may have too little fuel pressure. Like stated above I would find a way to view fuel pressure while you're driving it. You can buy a cheap transducer off eBay and hook it up to the power logger if you're using a power logger.


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Those rail gauges are known to be inaccurate so you may have too little fuel pressure. Like stated above I would find a way to view fuel pressure while you're driving it. You can buy a cheap transducer off eBay and hook it up to the power logger if you're using a power logger.


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No power logger. I'll start with resetting the ecm after work tomorrow. I'll let you know how it's goes. Thank you for your help
 
What are the AF, BL and INT numbers on your scan master at idle? That might help us understand what's going on with fuel system.


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I do not remember off the top of my head. After work I'll start car the car up and let it warm up all the way. I'll check AF, BL and INT before I reset ecm.
 
What are the AF, BL and INT numbers on your scan master at idle? That might help us understand what's going on with fuel system.


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Hey there Scott, I didn't get a chance to reset pcm today, had a long day. I did start the car after I got home, started right up ran for about a minute then died. Crank but no start after that. Gave it a couple minutes then I started it again but this time with a generous amount of throttle. Idled for some time after that before stalling again. It seems to acting like if I had just installed a new chip in it and it's having to re learn the parameters. My thought is after I initially put the new maf in it didn't have troubles because I hadn't changed the FP. Once I changed that though along with having a new maf it seems like the car is trying to re learn all the parameters again while on the fly. When I first put the new maf in the IAC hovered around 31. Once I started adjusting FP it got out of whack. When I checked it again today once I got it to run for a bit the IAC was off the charts at 106. My AF was at 04 my BL was at 141 (I had seen it around 128-135 when initially put the new maf in without other adjustments) INT was bouncing from 121-134. So tomorrow I'm going to restart the pcm and go from there. I did get s chance to get the FP to 43 vac Line off before car stalled again.
 
How long will it hold pressure on the rail key off? Sounds like a bad fuel pressure regulator
 
After changing the MAF and fuel pressure you might want to start by reseting the computer by disconnecting the power for a minute. Not sure who's chip you're using but if it learned your fuel curve at 60 PSI and now it's 43 that will make a difference. I would also start by checking TPS and IAC numbers after ECM is reset. You may have a fuel pump issue but I would start with the ECM. Do you have a fuel pressure gauge you can see while driving or are you logging Fuel pressure?


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^^^^^ This.

He says he has a FP gauge at the end of the rail, but that does him no good when the car is WOT.

I don't see a Scanmaster in his sig, nor the mention of one but it looks as if you have one. If you don't, this should be your first modification, period.

No offence, but it looks like a lot of "I'll just run this thing by the seat of my pants until she blows" going on here.... If this is you're first outreach, back the heck off of your car, now.

It could get costly, real quick.


But I could be wrong.....
 
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You are saying it ran good with your rail gauge showing 60 PSI with the vac line off? Those rail gauges are known for not being accurate. I would buy or borrow a fuel pressure gauge you can see inside your car to make sure your fuel pressure is rising properly. I have a gauge on my pillar because running a turbo engine too lean can get costly. I think it's a good investment but the auto parts stores sell a generic gauge with a long enough hose so you can tape the gauge to your windshield wiper arm. BL at 141 the computer is adding fuel. Maybe the FPR isn't working properly since 43+17 would give you enough fuel at WOT but would make things difficult on the ECM. I had a fuel pump go bad and the gauge read in the low 20 psi range. It had the same issues you describe with starting only with the throttle body open and would stall. Since I could see the gauge was reading low I tried adjusting the fuel pressure regulator and could only get lower pressure as the pump finally died completely. You say the car ran good and you had 60 psi with the line off so I'm not sure your pump is bad. Pumps I've seen go bad would not build pressure 1:1 under load with boost. Since you seem to be able to adjust yours and indicate its capable of high pressure (at least at idle) I'm not sure that's your issue. It seems more likely your gauge is not accurate with the car stalling after it ran good with no knock at WOT until the pressure was turned down. Do you have any vacuum leaks? Did you replace the MAF with the same size and style you had? Are you using a MAF translator? Where do you live? Maybe someone is close to you that can help you.




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You are saying it ran good with your rail gauge showing 60 PSI with the vac line off? Those rail gauges are known for not being accurate. I would buy or borrow a fuel pressure gauge you can see inside your car to make sure your fuel pressure is rising properly. I have a gauge on my pillar because running a turbo engine too lean can get costly. I think it's a good investment but the auto parts stores sell a generic gauge with a long enough hose so you can tape the gauge to your windshield wiper arm. BL at 141 the computer is adding fuel. Maybe the FPR isn't working properly since 43+17 would give you enough fuel at WOT but would make things difficult on the ECM. I had a fuel pump go bad and the gauge read in the low 20 psi range. It had the same issues you describe with starting only with the throttle body open and would stall. Since I could see the gauge was reading low I tried adjusting the fuel pressure regulator and could only get lower pressure as the pump finally died completely. You say the car ran good and you had 60 psi with the line off so I'm not sure your pump is bad. Pumps I've seen go bad would not build pressure 1:1 under load with boost. Since you seem to be able to adjust yours and indicate its capable of high pressure (at least at idle) I'm not sure that's your issue. It seems more likely your gauge is not accurate with the car stalling after it ran good with no knock at WOT until the pressure was turned down. Do you have any vacuum leaks? Did you replace the MAF with the same size and style you had? Are you using a MAF translator? Where do you live? Maybe someone is close to you that can help you.




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Could not find any vac leaks, I took starting fluid and sprayed around all the vac lines, found no leaks. They are both lt1 MAFS but I think the one that was in there before was a hogged out lt1 maf. And yes I do have a translator. I'm in st.louis mo. I'll go grab one of those gauges from autozone today, now do you think I should try that before resetting pcm, or should I do both? Also just in case this happens, if I turn the FP back up to 60 line off and car still does the same thing, then what do you think that could indicate.
 
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^^^^^ This.

He says he has a FP gauge at the end of the rail, but that does him no good when the car is WOT.

I don't see a Scanmaster in his sig, nor the mention of one but it looks as if you have one. If you don't, this should be your first modification, period.

No offence, but it looks like a lot of "I'll just run this thing by the seat of my pants until she blows" going on here.... If this is you're first outreach, back the heck off of your car, now.

It could get costly, real quick.


But I could be wrong.....
I do have a scan master
 
^^^^^ This.

He says he has a FP gauge at the end of the rail, but that does him no good when the car is WOT.

I don't see a Scanmaster in his sig, nor the mention of one but it looks as if you have one. If you don't, this should be your first modification, period.

No offence, but it looks like a lot of "I'll just run this thing by the seat of my pants until she blows" going on here.... If this is you're first outreach, back the heck off of your car, now.

It could get costly, real quick.


But I could be wrong.....
No not at all. I have owned the car for a year now, that was my 2nd time actually going wot and hitting full boost in this car.
 
How long will it hold pressure on the rail key off? Sounds like a bad fuel pressure regulator
I checked that yesterday after the car has stalled out. There was some bleed off, 5-6psi maybe but was still above 40psj after 5 min of shut off.
 
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