I know not another backfire through intake 1976 280z I think I tested it all

[cozye]

Intake vacuum will pull fuel out of the injectors a little as well, you can't simulate that easily. I think you are heading down the wrong path with fuel flow diagnosis. I spent 6 months diagnosing this same type of issue, as did fast woman. Your fuel pressure is in spec, if you had a flow issue it would drop below spec when running.

Any fitting on the manifold will work for checking vacuum. I use a t fitting off the fpr vacuum line. Brake booster is a good source if you have a fitting that works for that.

Do a compression test. While I have arrived at the same conclusion as fast woman about ecu drift, I also have another theory but I need more examples to make it plausible

[black gold man]

Why dose running with the timing advanced beyond factory specification control

the intake pop?

[FastWoman]

Eric, I did do a compression check. I got about 185 across all six, dry. What's your theory?

Black gold, the intake pop occurs because an overly lean mixture can burn extremely slowly -- so slowly that there's still a bit of fire after the end of the exhaust stroke and into the beginning of the next intake stroke, thus detinating the fuel/air in the intake manifold. If you advance the timing, you give the mixture a bit longer to burn before the next intake stroke; however, this addresses the symptom, not the cause.

Racebird, the injectors should put out a nicely atomized spray pattern, not a stream. Perhaps you're due for some new injectors. I use a set of Standard Ignition injectors I bought off of Ebay for $150. The rubber hoses are a bit short and need replacement, but otherwise they work fine. The same vender still sells them there.

I like to measure vacuum at the nipple that feeds the HVAC control systems -- the little nipple next to the brake's power booster vacuum line.

Edited February 3, 2012 by FastWoman

[FastWoman]

Er... Eric, my memory was a bit faulty. According to my service records...

" Took dry/warm compression, 10 strokes. #'s 1-6 = 165, 165, 168, 173, 165, 168."

I guess that's reasonable for an engine with 173k miles on it.

[racebird1]

I did some reserch on vacuum specs and I will do a comp and vac test Saturday and see how it turns out. I am leaning on low vac from a leak somewhere or the injectors not putting out enough fuel. The injectord I pulled off the other Z are a bit ragged on the outside and won't due for my restoration. If my next tests come out ok then I will get a new set of injectors. If this doesn't work I will need to look into the ECU or MAF.

[cozye]

My alternate theory was that ring wear caused reduce vacuum and that the lean condition could be caused by less fuel being pulled from the fuel rail. It's a long shot, but several things keep bothering me. One, not all people with 280's have the issue. Two, trying different ecu's doesnt help. Three, even when correcting the fuel mixture the vacuum is still a tad low compared to other cars that do not have the issue. The best I can get mine is around17.5

My compression results were in the low 160's which is a bit low iirc?

It's Also entirely possible that the rings could seal better on compression due to the positive pressure, but leak more on the intake stroke due to the negative pressure?

Edited February 4, 2012 by cozye

[racebird1]

Well I got arround to cheching the vacuum and compression and was dissapointed with the results. The Vac was steady about 13" at 800 rpm and my compression was only around 135 to 140 on all 6. Naturally this is not what I was hoping for. This car aside from the Intake backfire runs pretty decent and it does not smoke at all. The plugs all had light gray deposits on them. So is there anything else besides bad or stuck that can cause this? Any thoughts on what my next move is here? I reallly want this car to be on the road this year.

[cozye]

If you are 100% sure there are no vacuum leaks and your AFM hasnt been tampered with, and your throttle switch is working properly, then put resistance in line with the water temp circuit. The best way is to use a 5k linear potentiometer. The kind you have to use a small screw driver to adjust. Then solder two bullet connectors on it and you can hook right in where the harness connects. Use your vacuum gauge to get a close tune, it should get up to about 17 or so. Then use driving the car and plug reading to get it better. I found that when I tuned it for max vacuum, it was actually a little too rich and I had a bad stutter at off throttle. I've got mine running good now. Drives perfect.

[cozye]

Other than that, I think a rebuild is on order. I'm contemplating doing mine next fall

[racebird1]

Well I may need to redo my compression test I had the engine warmed up and all 6 plugs out but I did not hold the throttle open. I think I will test a few again with the throttle open.

[cozye]

I won't make that big of a difference. I did the same thing. Don't get your hopes up

[racebird1]

I re did the compression test on number 2 and 5 with throttle wide open and air filter out and you were right absolutely no difference. Then I did a wet test on those two cylinders. On #5 I put about three squirts of 5-20w oil in and tested again and got 147 psi then I proceeded to #2 and put about 6 or 7 pumps in and checked it and got 190 psi. What a huge difference. The afm is not the original one so It is possible that it not set to factory and I tested the vacuum while blocking off other vacuum ports such as brake booster and heater controls and did not see any changes. Now this engine has only about 2 to 3 hours of run time on it after sitting since 1988. The inside of the valve cover was clean but is it possible that the rings may be a bit sticky and is there any way to tell or can anything be done about it?