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

[racebird1]

I hope someone can help a little here. 1976 stock 280z backfire in intake when accelerating Not too bad but still it does it . I did all the FI tests timing is at reccomended 8 deg btdc at 800 rpm, checked for vac leaks, fuel press at constant 32 spikes briefly when reved to 40 and holds 30 for a while after shut off and return line puts out about 3 liters a minute while running. I do believe that it backfired more than usual while I had the return line running into the container. It has new wires and plugs. I did have the injectors and fuel rail out, while it was out I pressurized the rail untill the injector cleaner came out of the return line and I ran the injector cleaner through all of them them and they all seemed to work normally however they only shoot a steady stream with the cleaner and I don't know if it is a little thicker or is a stream normal. they did not produce a spray pattern but then again the cleaner used may be thicker than gas. They all put out about the same amount during a timed period. It may have backfired a little more than normal while I had the return line running into the container. when I had the injectors out I did not know there are filters in them so I did not check that or clean them. Does any body have any thoughts here?

[cozye]

It's common on the 280. The are several very lengthy threads with detailed next steps, ie. ruling out vacuum leak, etc.. Once you go through everything there is a last resort fix but you must make sure it's not an air flow problem or other fi issue first. I think it's safe to say you have ruled out fuel flow as the issue.

[240260280z]

Quick and dirty check for vacuum leaks is to set your idle really low then spray carb cleaner around all fittings margins and hoses.If there is a leak, the engine will suck the carb cleaner through the leak and it will increase in idle speed.

One strange problem that happened earlier this year was the casting between the egr and the intake plenum. It rotted through messing up the fuel air mix.for cylinders 5 and 6. It happened to HealeyZ on this forum

Loose FI connectors, failing ignition module, problem with distributor, worn cap or rotor, bad spark plug wires, etc. can also be culprits.

Edited February 1, 2012 by Blue

[FastWoman]

Blue, I do remember the weird EGR to intake hole in "argneist's" intake manifold (a '78, as I recall), but I don't remember that happening with HealeyZ. Now TWO cases? Interesting.

Racebird, Cozye and I went through all of this in a long thread of mine named "purs like a kitten," which you can search. We both concluded, as have others, that the ECU can drift in its operational properties over the decades. This would make it like almost any other elderly analog electronic device I've had the pleasure of servicing.

[240260280z]

My mistake

[FastWoman]

Phew! One case is a fluke. Two would be troubling.

[racebird1]

I didn't mention that the cap and rotor are new and I tried another ECU and both units the car ran exactly the same. It held 20 pis on the fuel pressure gauge over night. 32 psi is a little low and I wonder if that would cause it. Supposedly lean mixture would cause this intake backfire. So I am leaning towards slightly low pressure and maybe partially cloged injector screens. I have another 76 z that I pulled the fuel rain and injectors out of today so I can play with them and see how they spray compared to what the ones I used did. I read a post where someone pinched off the fuel return line to make sure the pump can pump more that the pressure needed.

[cozye]

It's not your fuel pressure. You can unplug the vacuum line to the fpr if you want to witness the fuel pump put out more pressure.

What is your vacuum reading and your compression test results?

[Captain Obvious]

Phew! One case is a fluke. Two would be troubling.

Well then... Be troubled. I'll go on record as number two.

I had a 77 intake that had an erosion issue directly underneath the EGR valve. I don't know if it actually made connection between the two areas of the manifold, but it was so bad that you could see cracking and peeling/flaking on the OUTSIDE of the casting under the EGR.

I gave that manifold to another member here for experimental surgery and/or practice. I could dig through my notes and get in touch with him if you would like. Maybe he would take some pics of that area (if it's still attached). He said something about making a non-EGR intake out of it.

[FastWoman]

Yikes! Yeah, argneist just disconnected the EGR as well. Of course this is thread drift, so we can leave it at that.

Racebird, I tried a different ECU as well, and I found my car ran exactly the same way. It took quite a lot of diagnosis to determine both ECUs had drifted lean in exactly the same way. My chief suspect was the AFM, but it checked out in every possible way, and indeed it operated exactly the same as another AFM I ordered in.

How are you measuring your fuel pressure? You should measure it with the vacuum line disconnected to get the proper static reading. A reading of 32 would probably be about right for a vacuum reading of 10 in Hg or so.

[cozye]

Sarah, did you ever do a compression check on yours ? What were the numbers?

[racebird1]

I measured the fuel pressure by tying the gauge in just after the filter which is also new. That was with the vac line still attached to the FPR. I didn't check it with the line off. Where is the best port to test the Vaccum? Is there a way to test an ECU to see if it has drifted away from specs? I took a complete fuel rail with injectors off another 280z and connected it to my fuel line and cranked the engine to get the pump to run and got even lower pressure on that setup (25psi) and I did get some return from the return line but not quite as much. I checked the spray pattern on those injectors and got the same as the ones that are on the car. Basicaly a good strong steady stream but not really a spray. Does anybody know how much volume one should spray for lets say a minute at the 36 psi? I never checked the compression because this car only has 41,000 original miles on it although it was sitting for 25 years so I probably should check it anyway. This car is just about completely restored and I just can't get this bug out.