Scottie-GNZ Posted January 28, 2001 Share Posted January 28, 2001 Ever had one of those days that made you want to cry and laugh at the same time? Spent most of the afternoon with a buddy trying to get to the bottom of my fuel pressure problem. I think we finally found it and in the process made some eye-opening discoveries. We have concluded that the problem is the FPR. This is a brand new replacement that the vendor sent me. With the fuel pump running, we attached a Mity-vac pump to the vacuum port of the FPR and applied pressure which should simulate boost. We were not sure if this was going work so we tried it on another GN and got a steady increase of 1# fuel pressure per 1# "boost" applied by the Mity-vac. With mine, when we applied 20# pressure from the pump, the FPR was registering an increase of 24# . That explains why the car ran so rich Wed night. I am running an open loop mode chip that expects the pressure to be set at 46# at idle, vacuum line off. Dont know why I was reluctant to beleive I could have gotten 2 brand new FPRs and have both be defective. Then proceeded to check every connection for leaks with the Mity-vac and made 2 frightening discoveries. My boost gauge is badly miscalculated. At 20# on the gauge, it is reading 4# high. This is a gauge from a highly reputable aircraft instrument manufactuer who claims to calibrate their gauges carefully. Imagine if it was reading 4# low . To double check, we did the same test on his GN. Continued checking and then discovered an alsmost new PCV (Purolator) was not functional, which means I was dumping boost through that huge hose into my crankcase!!! The consolation is that if the gauge is really off by 4# at 20# boost, that means when I ran 11.50 @ 120.5 and thought I was running 21# boost was really only 17+# boost. In addition, how much boost was I losing pass the PCV? Guess I will be laid up for a little while but think I have finally herded those cats and have something to look forward to when I get it all right. I would highly recommend getting a Mity-vac and testing every hose. You might be surprised what you will find. Invest in a big bag of zipties. ------------------ Scottie 71 240GN-Z Scottie's GNZ [This message has been edited by Scottie-GNZ (edited January 27, 2001).] [This message has been edited by Scottie-GNZ (edited January 27, 2001).] Quote Link to comment Share on other sites More sharing options...
Guest pauli Posted January 28, 2001 Share Posted January 28, 2001 damn... at least none of those problems were catastrophic. really, if you've got to have problems with a turbo motor, there are worse situations than running rich and getting less boost than you think... although i'm probably wrong, would fixing the boost leak compensate for the rich mixture, or would it not help anything? Quote Link to comment Share on other sites More sharing options...
pparaska Posted January 28, 2001 Share Posted January 28, 2001 Glad to hear you're getting to the bottom of it. The Holley carb-type FPRs are hokey as hell as well. I hate gage manufacturers. I don't know where you got yours, but I know the Autometer stuff can be WAY out of calibration. There kind of pedestrian, so I'm not too surprised, but hell, $65 for an electric temp gage and its off (high at least) by 20F? I bought Mike Kelly's old Street-Comp water temp gage as part of a package deal. I checked it by getting a clean powersupply, my trusty Fluke VOM, a pot of water on the stove (the wife loves this) a laboratory thermometer and a bunch of clip leads. Well, from 180-210, the thing read 20F too high. I figured, hey, it was used, maybe that's it. So I bought a new Pro-Comp electric water temp gage to match the oil filled mechanical Pro-Comp oil pressure gage I got from Mike. Did the same test, and IT'S about 20 degrees too high. I know what your thinking - Pete's laboratory thremometer is off. Nope, I tried another, and then tested against Mother Nature herself. The thermometers read 212F with the water at a full boil! (I'm pretty close to sea level, so water DOES boil in my kitchen very close to 212F) The freaking gage/sender is the problem. I tried varying the supply voltage from 12-14 volts, and it made little difference. Anyway, I broke out the pencil, paper, and Ohm's Law and came up with a pair of resistors to put in series and then parallel to the gage to get it to read dead on from 180 to 210. It's way high below that, but I don't care. Past 212, the math says it should be fine. Basicaly, I had to change the sensitivity and offset of the gage/resistor network to get it to work in the 180-210 range. Not perfect, but workable. Mike's new Autometer gages for his car are on my bench now waiting for me to "calibrate" them. Too bad there's no easy way to adjust them. You have to uncrimp the bezels to make any kind of adjustment, so I'm sticking with putting them in a resistor network. Next time, I'm going to use a mechanical temp gage, as those are supposedly more accurate. BTW, I also bought a 10-70 ohm Ford type Pro-Comp Fuel level gage, and worked out a way to get it to read full and empty when the tank is such. The Older FOrd fuel level senders are pretty close to the same range of resistance as the 240Z/260Z/280Z ones. Waiting to get a full tank of gas before I can finalize it. [This message has been edited by pparaska (edited January 27, 2001).] [This message has been edited by pparaska (edited January 27, 2001).] Quote Link to comment Share on other sites More sharing options...
Drax240z Posted January 28, 2001 Share Posted January 28, 2001 Wow, startling discoveries pete. I'm really interested in hearing about that ford fuel level sender swap thingy! ------------------ Drax240z 1972 240z - L28TURBO transplant on the way! Drax's 72 240Z Turbo Quote Link to comment Share on other sites More sharing options...
Guest Anonymous Posted January 28, 2001 Share Posted January 28, 2001 From what I read somewhere and I thought it was my Datsun chassis manual, although I can't find it now, it may have been Haines manual, the reading is 10-90 ohms for Datsuns sender. Amusing story (sort of), I bought the Autometer Ultra light gauges to replace water and Fuel (and oil when I do my swap soon). I bought the GM sender variety going on the above info knowing it would be off but close. Well after wiring it up, Its reading 1/2 tank which I think is about what was in it before I swapped out the gauge. I go to the station and fill it up. I get in, turn it on and it reads 1/8th tank. Apparently its bass ackwards from the GM sender, I hooked the wires up exactly as Nissan had them, and since its measuring resistance I didn't think swapping them would help (Obviously I'm no electrical engineer). So, for the time being, in my car, when it starts getting full, I need to get worried. I'm gonna change to a fuel cell at some point with a GM sender anyway, or weld up a custom tank so it didn't bother me much. Just a final word, if you want to use the Autometer 2.5" gauges, simply remove the face on the gauge your removing, and the Autometer will drop right in the Datsun gauge frame. Secure with Autometers gauge holders in the back, fashion a simple bracket to hold the gauge to the gauge mount and its done. (I used heavy wire for the gauge mount and it worked just fine.) Sorry for the long post... Lone [This message has been edited by lonehdrider (edited January 27, 2001).] Quote Link to comment Share on other sites More sharing options...
Guest RON JONES Posted January 28, 2001 Share Posted January 28, 2001 Well Scottie,it seems that once you have this problem fixed,you may well see that 10 second time slip.I hope you have it fixed in time for when I'm down there. Quote Link to comment Share on other sites More sharing options...
John Scott Posted January 28, 2001 Share Posted January 28, 2001 Scottie, as usual, I'm impressed with your professional systematic approach to a problem. It sounds like you're on your way to better time slips. Your pump reading of 24 lbs has me interested. Was this reading over and above a baseline pressure, or the actual pump pressure? Since the rule of thumb for SC'ed carburetion is 7 psi over max boost, the 24 lb is about perfect for 16-17 lbs boost. Sorry I can't remember, but what fuel pump/ regulator are you running? Prices? Quote Link to comment Share on other sites More sharing options...
TimZ Posted January 28, 2001 Share Posted January 28, 2001 Well, don't feel too bad - at least it was off in the safe direction - would've been worse if the fuel pressure had been too low . One thought - I agree that it's suspicious that you were able to verify a 1:1 relationship on another vehicle, but it's also suspicious that both the gauge and the FPR showed the same error (4psi high) on your car. Are you sure you trust your mityvac gauge? I'm kind of in the same boat - I've been trying to do a conversion from kPa to relative gauge pressure (+psi, -inHg) for my datalogger. The math conversion is simple enough, but the readings are about 10% low when I compare to my mityvac. Now, I'm just not sure which gauge to trust. All of my test pressure gauges are setup to read relative pressure (including the mityvac), and have a 'dead zone' around 0psi, presumably to allow for differences in atmospheric pressure. Anybody have a source for a truly accurate pressure gauge? [This message has been edited by TimZ (edited January 28, 2001).] Quote Link to comment Share on other sites More sharing options...
Scottie-GNZ Posted January 28, 2001 Author Share Posted January 28, 2001 Tim and John, the points you raise are ones that we considered but feel confident about the results. The pressure measured at the FPR was done with a very accurate top of the line guage purchased from Grainger. When it registered +4 compared to the mityvac, we performed the same test on his and the mityvac and pressure gauge was dead-on 1 for 1 up to 20#. Cannot say for sure how accurate his boost gauge is, but it matched the mityvac within fractions. John, re your question, this very topic is in another post. With my FPR set to 46# at idle, vacuum off, it should register 66# at 20# boost. This is a situation I have been battling for about 2 months and my frustrations just came to a head. I am certain my blown head gasket was the result of the beginnings of the FPR going bad. Before the blown gasket, I had 3 runs at 21# on the guage with the O2s very rich and then POP. You normally should not blow a head gasket with ~17# boost and 112-octane unless your fuel volume dropped tremendously. BTW, my buddy ran his GN Thurs night and we have the same setup except he is running a stock ported turbo with a larger exhaust housing. He ran 11.92 @ 111.77. Does not sound impressive until you realize his car weighs in at 3,750lbs and the HP formula spits out 430RWHP. My immediate goal is to match that HP number which should put me in the 10.80s. ------------------ Scottie 71 240GN-Z Scottie's GNZ Quote Link to comment Share on other sites More sharing options...
Mike kZ Posted January 29, 2001 Share Posted January 29, 2001 Originally posted by pparaska: BTW, I also bought a 10-70 ohm Ford type Pro-Comp Fuel level gage, and worked out a way to get it to read full and empty when the tank is such. The Older FOrd fuel level senders are pretty close to the same range of resistance as the 240Z/260Z/280Z ones. Waiting to get a full tank of gas before I can finalize it. Pete, let us know what you find!!! I have a Phantom Auto Meter Ford fuel gauge. ------------------ http://members.tripod.com/~SnowSurfer/mikekz1.html Quote Link to comment Share on other sites More sharing options...
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