Guys - I have a stumbly/shakey idle, maybe a miss. I did a AFR leak down test and ran the car and shut it off to observe the Fuel pressure guage. It went from 41 to 23 with in about 6 minutes and held at 23. I checked to see if fuel was coming from the vacuum line and it was not. I also did the old school water spray bottle test on the headers and the number 8 seemed to be cooler as the water would run down it a bit. Not sure if the 2 scenarios are related. Any input would be appreciated. I know some of you may say "leaky injectors" for a possibilty. I am on my second set of brand new aftermarket injectors chasing my idle issues so I can't believe 2 sets would have leaked. Could a bad AFR (broken spring etc) cause any of this?

 

While it might not be the perfect, factory specified, idealist-approved condition, holding anything more than 20 pounds for more than a few minutes is more than adequate. Fuel pump check valves can leak down that much, and it isn't a problem. If you understand fluid dynamics, you'll recognize just how little volume is actually leaking through or back when you consider the volume of the closed system and pressure change over time. Operationally, when the fuel pump is running it will not even notice that little bit of leakage (on the order of 0.25-0.33 cc/min). Of course, if that fuel is leaking into one cylinder it may make a slight difference in restart after heat soak (on that one cylinder) but distributing 1.5-2 cc of fuel through the entire engine in six minutes isn't going to be a problem.

As for the #8 cylinder being cold, that could be a fuel delivery problem in that single cylinder, or an ignition issue, compession problem, valve timing problem (some cams come ground with one lobe off-time), or other mechanical issue. I wouldn't consider it as being a general fuel pressure or delivery problem.

 

Vader - Thanks, good info. Some tests on number 8 that I can think of would be to check for spark through the wire and maybe for power at the injector wire. As for compression, I had the block rebuilt and I had the problem previously so I would think compression is OK. I can't rule out that the cam is screwed up but I can say that it is an LPE unit which means it should have been quality. Is it common for a cam to be out of spec from day one? I did just replace the dizzy and re-timed it with out luck. Can you suggest any other tests?

 

clamp the return line by the tank it should be rubber. try your test again by appyling 12volts to G terminal under the dash ony for a second just to get the psi to 45. any high the line could burst..check to see if it leaks down again. if it does then its your checkvalve or the pulsator in tank that connected the fuel pump to the lines. the pulsator is a slip on part with diaphrams to help ease the pulsing from the injectors. when my pump took a dump, i seen the pulsator not even connected. i replaced with neoprene hose with stainless steel clamps. (came with pump from the stealership) most likley its that. but then again the miss could be something else like an injector connector not press completly on the injector. whats your gap on the plugs, and try pulling the 8th plug and check to see what color it is? (red, black, white) etc check the gap and did you replace the whole dizzy, is everything new in it? maybe your tps is failing. btw. my fuel pump doesnt hold any pressure after turned off. (because the hose on the fuel pump is leaking because the stainless steel clamps i used were self tightening(dam it) i need to go back and use the clamps that require a screwdriver. but besides that, i dont have misses or nothing. its most likely and ignition problem. when was the last time u changed the inline filter.