JohnC Posted November 11, 2005 Report Share Posted November 11, 2005 Here's a question for those who have a fuel pressure gauge plumbed into their PI system: Do you see a different fuel pressure when the car is hot compared to when it's cold? I have a pressure gauge fitted to my MU, in the position used for a PRV in some rare cases (top of the MU, blank face on most MUs). I set the PRV on the car, and I've noticed that the pressure after a run, when everything is hot, is about 5-10psi lower than when cold. If I set 105psi when cold, I get a fraction under 100psi when hot. If I set 110 cold, I get just under 105 when hot, etc. In all cases the reading is taken with the ignition on, but engine not running. It makes no difference if the tank is full or nearly empty (petrol in tank warms up much faster if the tank's nearly empty), although all tests have been carried out in hot weather (high 20s). What I'd like to know is if this is normal, or if it's evidence of an underlying problem. This may of course be a case of "what you don't know won't hurt you" - set the correct pressure on the bench or when cold, and then leave well alone! I'd really like to hear from anyone who has a pressure gauge installed - I suspect that this is not something that will show up on bench tests; In fact my PRV, pump and MU have all been bench tested & calibrated in the past year, and all appeared fine. Thanks in advance, John Quote Link to post Share on other sites
tr4-thomas Posted November 11, 2005 Report Share Posted November 11, 2005 hi John my friends tr6 shows the same readings. the car is running very fine and smooth, so we think it is normal. regards Thomas from germany Quote Link to post Share on other sites
JohnC Posted November 11, 2005 Author Report Share Posted November 11, 2005 Thanks Thomas. Out of interest, where does your friend have the gauge mounted? Is it in line with the MU (i.e. a 'T' fitting), or somewhere else? Cheers, John Quote Link to post Share on other sites
tr4-thomas Posted November 14, 2005 Report Share Posted November 14, 2005 hi John it is fittet in line with a "t-fitting" . the gauge is mounted behind the radio console .its not really good to see the reading when your driving, but for testing purposes its ok. a hose of about 1 meter is connecting the gauge to the t-fitting. there is also a self-closing valve mountet to the t-fitting, so its easy to undo the hose with the gauge. all this pieces are originally hydraulic-components. hope that helps regards Thomas from germany Quote Link to post Share on other sites
Guest stevephillips Posted November 15, 2005 Report Share Posted November 15, 2005 John I have a 72 TR6 and fitted a pressure pump about 8 years ago as I was having the old cavitaion problems and the fitted pump was the only real way to know at that time that the pump was the issue. A Bosch conversion is now fitted and whilst it has never given me a problem like the lucas since it is kind of reassuring to still see it reading 105 when I am tuninig or looking under the lid. I have fitted the gauge above the passenger well on the engine bay side of the bulkhead and it has work for the last 8 years (see attached picture you can just see the bezel at the bottom of the screen). I tee off to the gauge between the MU and the feeder pipe into it. As for pressure hot and cold I know that when when I fist turn the ignition and leave the pump running to get the pressure up it is reading 108 psi and when running it drops down a couple of psi on tickover (though once the engine is running the needle vibrates a bit so this reading is not exact. As for the hot temperature with the pump running and engine hot but stopped I am not sure. Fortunately I am off for a 600 mile run this weekend (300 miles each way) so if you can be patient I will take a look and let you know the results. Regards Steve Quote Link to post Share on other sites
badshead Posted November 15, 2005 Report Share Posted November 15, 2005 though once the engine is running the needle vibrates a bit so this reading is not exact. Steve You could always swop the pressure gauge for one filled with glycerine to steady the needle. The 'minimess' system is a neat solution if you don't want to keep all the plumbing and gauge permanently connected. Comprises a microbore hose which can be connected/disconnected from a special T adaptor by hand with no fuel spill or spanners required. Being microbore it has the advantage of easily being routed (temporarily or permanently) through the bulkhead if you want to monitor the fuel pressure while driving. Quote Link to post Share on other sites
JohnC Posted November 16, 2005 Author Report Share Posted November 16, 2005 Steve, I would appreciate your findings once the engine is hot. I think I'm going to see If I can source the microbore/minimess system as well. Cheers, John Quote Link to post Share on other sites
badshead Posted November 16, 2005 Report Share Posted November 16, 2005 John Most hydraulics suppliers like RS Components, Pirtek etc will have minimess components - I guess you'll have your own suppliers in Oz. Try Hydrotechnik - Minimess Test Points for info. Part no SNA02 will connect straight into the supply hose/MU connection. Buy yourself a pressure gauge (part no 9801-01-00.10) and microbore hose (part no S100-AC-FA-00.63 - or other length) and you've got yourself the full kit for a fraction of the price they regularly go for on Ebay. Quote Link to post Share on other sites
Guest stevephillips Posted November 17, 2005 Report Share Posted November 17, 2005 John Will do but we are having a bit of a cold snap here but hopefully this will not effect my results. I have two trips in total there an dbak so I should be able to get some resonable results and 300 miles is more than enough to get a steady temperature. I have also refitted the thermostat today to make sure that I get an even temperature. What did you think about the position I have the gauge? I find that this position works well and easy to fit. Good luck with the micorbore Badshead, I will think about the Glycerine but as this is more or a quick visual I think it will be lower down on the list of TTD. I am sure it will give up one day and then would be the most practical time. Regards Steve Quote Link to post Share on other sites
Guest stevephillips Posted November 22, 2005 Report Share Posted November 22, 2005 John Went up noth as promised and these were the results outward Start of trip pump only (cold) 114 Start of trip pump and engine (cold) 109 End of trip pump only (hot) 112 End of trip pump and engine (hot) 108 inbound Start of trip pump only (cold) 113 Start of trip pump and engine (cold) 110 End of trip pump only (hot) 112 End of trip pump and engine (hot) 109 Would suggest that it does not really matter too much whether hot or cold. Hope this helps. The brute went like a dream and I was a very smilie chappie all the way. If you need anything else let me know. Steve Quote Link to post Share on other sites
Peter Douglas Winn Posted November 22, 2005 Report Share Posted November 22, 2005 Realising that most measuring kits have an an accuracy of +/- a brick, I think this is dammed good. Quote Link to post Share on other sites
jonlar Posted November 22, 2005 Report Share Posted November 22, 2005 Hydrotechnik claim 1.6% accuracy. "Accuracy is Industry standard +/- 1.6% of full scale." Quote Link to post Share on other sites
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