scr0tum

I have visited this site over the past few years but have just recently registered. I'm having problems with my fuel system, this will take a while to explain but the more detail the better, right?

I have a 1993 Z28. The bottom end has around 85-90k miles on it and the top end has around 25-30k on it. I have 32lb injectors with an LT4 conversion kit, shorty headers gutted cat, 58mm TB, CAI, AFPR set to 50psi at no vacuum.

A few months ago I noticed my car acting up... hesitating when i romp on it, stalling at lights when i hit the gas to take off, hard to start in the morning, after i start the car it has 0 power until the 02 sensors warm up about a minute (i can rev it to 2500 rpms and as soon as i start to engage the clutch it bogs down and dies).

I chocked this up to the fact i ran around for 6 months plus missing 3 header bolts (i keep losing those damn things) and figured i had blown an 02 sensor out. So i replaced both 02 sensors 2 weekends ago. That didn't change anything at all. Then one day on the way to work i caught something move out of the corner of my eye when my car hesitated again, it was my fuel pressure gauge on my hood. I noticed that if I hit the gas pedal the fuel pressure would drop down to 40 psi (what it sits at under full vacuum engine decel) before it raised up to 50 (wot fuel pressure). So I was all excited to learn new info about why my car has been running like crap. Last weekend i changed out the fuel filter and it didn't affect it much, the needle on the fuel pressure gauge seems to be ever so slightly more responsive now but nothing on performance and i still get the fuel pressure drop. So tonight i decided i would see how the fuel pressure ran if i disconnected the vacuum line from the AFPR and it stayed at a rock solid 40psi even as i reved it a bit in the parking lot. I was really hoping that it would be the fuel pressure regulator but i'm not convinced it is now, i don't smell gas in the vacuum tube, it smells like something but not gas. The vacuum line isn't clogged at all and i don't really know where to do now. The only choices i really have are the AFPR or the fuel pump. I would assume that if the fuel pump was going it would be more catastrophic since i have had this system working perfectly fine since Jan 06.

I apologize for such a long post but i am hoping that i can get some advice from the great people on this board. Maybe some more tests that i can run or some other way to narrow it down. As far as my plans forward, i'm going to try and find the stock FPR (somewhere in my garage) and switch it out to see if that helps. If that doesn't help then i am going to need to change the pump i assume.

Final notes:
It doesn't have this problem all the time, sometimes it accels just fine. Since i got the car in June of 2005 it has never kept fuel pressure in the system when the engine is off (drops from 50psi to 0 in a second or 2 as soon as you shut off the key). Is it possible that the screen is clogged or something? Should i just start putting fuel system cleaner in the tank at every fill up?

Thanks so much in advance for the time it took you to read this and for any help you can provide.

Joe N

Injuneer

You indicate you have your AFPR set for 50psi with no vacuum, but when you pulled the vacuum line off, you only get 40psi. Doesn't that sort of tell you the fuel pressure is not adequate? If it was tuned for 32# injectors, operating at 50psi (flow 34.3 #/HR if originally rated at 43.5psi/GM or 36.2 #/HR if 39.15psi/SVO), the injectors have lost at least 10% of their flow capacity due to the 40psi no-vacuum pressure.

You need to figure out why you've lost so much fuel pressure. Pumps can wear out gradually.... failure is not necessarily "catastrophic".

Guest47904

From dead cold, the PCM takes longer than a minute to go into closed loop. Until it does, it doesn't matter what the O2s are doing, it ignores them. At cold start up until it goes into closed loop, the PCM richens the fuel mixture. You could have a condition that is rich to begin with and during warm up makes it worse.

Would be helpful to know what the fuel trims are doing after it goes into closed loop.

Wouldn't hurt to pull some plugs to see what they look like either. They can give you an indication of what's going on.

Slick-Z

Like what was said earlier my fuel pump went out gradually and since you beefed up the top end demanding more fuel then that might have finished your tired old pump out. Mine was running around 39 psi when i replaced it and it turned out not to be the case. I looked and when i turned the ignition key to turn the radio on but not the car the fuel pump automatically engages for one to three seconds building up the correct pressure. That is when i heard a leak from the rear turned out to be one of my lines that time. After i noticed it i started the car and went to the back and could hear the hissing sound from underneath. well anyhow the factory pumps run between 41 to 49 psi on average if its a 40 then the pump is probably worn out. You can also buy a fuel gauge that hooks up to your fuel rail for about forty dollars and hook it up to your fuel rail jump in the car turn the ignition key while not to start the car but to turn on the accessories. Go check your pump gauge reading its should stay at a steady psi. I checked my car once i knew i had a fuel leak and my psi fell almost automatically within five minutes. Well im writing a paragraph hope i was some help.

scr0tum

I may have typed it incorrectly, I will check again today. Thanks for the input. If this is the case though, wouldn't the problem point more towards the FPR than the fuel pump? If the pump was having a problem supplying adequate fuel at idle, then it probably wouldn't even run under load at all.

Thanks for the reply. I think it is weird that this only happens in the mornings and not when i leave work at night after my car sits for 9+ hours. As far as I know i can't get any data from the computer as it is OBD1. If that is incorrect then maybe I can get someone from a parts store to read it for me?

Slick-Z:
As I said, my fuel system doesn't hold pressure, as soon as I turn of my car or the pump stops priming it drops to 0psi in about 1 second.

Thanks for all the input guys.

Injuneer

You can get the same info from an OBD-I PCM as you can get from an OBD-II PCM, except for misfire count. You need an OBD-I scanner like the Auto Xray (available cheap, used on ebay). Or get TTS DataMaster scanning software for a laptop, and a cable from AKM Electronics. And you can flash the trouble codes on the SES light with a paperclip.

http://www.ttspowersystems.com/DataM...downloads.html

http://www.akmcables.com/

Courtesy of Shoebox:
http://shbox.com/1/dlc2.jpg

Interpret the data:
http://www.injuneer.com/ScanMast.html

You have a limited number of things that will cause this:
-faulty check valve in the fuel pump
-leaking injectors
-faulty fuel pressure regulator
-leaks in the system

scr0tum

Thanks for the data. The engine light comes on periodically while driving but goes back off, the only code that gets thrown is a "15" which has to do with the o2 sensor readings.

I did removed the vacuum line again this morning and I mis-typed before, with the vacuum line disconnected from the AFPR the fuel pressure stays at 50psi. It would make sense to me that the fuel pump was failing if the pressure dropped below 40psi, but it doesn't. Also this morning I did a little more testing of the engine shutting off when tapping the gas from idle. When I tap the accelerator pedal I can see the fuel pressure spike up (which is a good thing) but the engine rpms actually drop and it shuts off sometimes. I am wondering if maybe the vacuum is acting weird because of the intake system? I can't remember the last time i cleaned the air filter (i have a moroso intake and filter). I am wondering if a parts store would have a vacuum gauge for rent so that i can see if the fuel pressure changes are matching the vacuum level changes as they should.

scr0tum

I have had the problem longer than I've had my injectors, new injectors were installed in Dec/Jan of 05/06. I don't know of any leaks in the system, I get around 12-13 mpg in the city and around 17-20 on the interstate and the car dyno'd around 330-340 at the wheels so I figured the fuel mileage was on par with what it should be with 3.73 gears. I suppose both of the other options are feasible. Like I said a little earlier, the plan is to find the stock FPR in my garage somewhere and switch it out to see if anything happens, if it doesn't solve the problem i can only assume it needs a new fuel pump.

I have only ever put premium fuel in the car necessary as far as i'm concerned with a 10.8:1 SCR.

Injuneer

What brand is the AFPR?

scr0tum

I am not 100% sure, I think I remember it being an AEM. There is no marking visible but it has the adjustment screw and vacuum port look like the one on their site: http://www.aempower.com/ViewProduct.aspx?ProductID=135

I think I am going to try and drive it around the block after I get home tonight without the vacuum line connected to the AFPR to see if i can get the fuel pressure to drop below 50. Do you see any drawbacks in that? The system will always be running rich, so I figured it would be safe.

scr0tum

Can someone confirm that the information on Shoebox's site is correct for engine codes? http://shbox.com/1/Dtcs.htm

I could have sworn that the code 15 had something to do with oxygen sensors, but this says ECT sensor. If this is true I would assume this is the one located near the water pump and not the one in the head as the one in the drivers head is only used for the temp gauge and the one near the water pump is used in fuel calculations. I just replaced this recently as the connector on the sensor side broke and I could no longer plug in the sensor. If the sensor was open or shorted it would make the computer think the car was really hot or really cold and provide the incorrect amount of fuel, possibly causing the loss of power i see in the mornings. I believe every time i ran into this problem before was when the cars connection to the sensor had been severed.

While this is great and all, i don't think that it solves my fuel pressure problem.

Injuneer

Shoebox's code list is correct.

maddog05

could it be a simple gloged fuel filter?

scr0tum

I changed the fuel filter last weekend, it didn't seem very clogged as i easily blew some air through it.

I flashed the codes again when I got home tonight, I got a 15, 61 and 67. The two relating to the A/C system are strange as i've never seen them before and i haven't tried to use the AC in months. Does anyone happen to know what the threshold temperature for code 15 (engine temp sensor cold) is? I live in FL where the temp hasn't dropped below freezing as far as I know this year.

Another symptom i picked up tonigh: i pulled up to a light and looked down to verify my ears and noticed my engine was idling higher than normal, about 1400-1500 rpms according to the tach instead of the standard 900 to 1100. My idle is a bit high because of the LT4 hotcam i have in there. It seems as though the higher than normal idle and stall may be caused by a vacuum leak, but the fuel pressure drop i notice upon accel would indicate the opposite, increased vacuum. I stopped at the gas station tonight to fill up and added a bottle of fuel system cleaner. I am pretty stumped on this one.

Injuneer

The PCM sets code 15 based on the signal voltage. Basically, it sets the code for an open circuit (meaning the resistance of the sensor - or the total circuit - is over 100Kohms, indicating a temperature of -44*C/-47*F). Make sure the wire harness is plugged into the sensor and that the wire or the pigtail isn't damaged. Check the sensor resistance, per Shoebox's guidelines:

http://shbox.com/1/4th_gen_tech2.html#ect