I did disconnect the MAF sensor, but then the backfiring happens all the time, independent from throttle movement.

Thinking zebra?

 

With the vacuum on it runs lower at idle because it needs less fuel. What I am curious about is whether it holds pressure after shutdown. So lets say you turn it on and it jumps to 39, does it hold the pressure for a while?

Also, is the timing at 6 degrees? Have you checked that the harmonic balancer is not walking around on you? Sure, you get it right with the timing light but if the outer ring is moving, well....

 

Or your ECM is compensating. With engine on and everything off after it has achieved stable operating temp, the IAC counts should be about 20-30 stable

 

It drops rather quickly to about 20psi, then it slowly drops down further.

Timing is at 6degrees. How can check the harmonic balancer?

 

Can injectors make that much noise?

 

I believe they say if you are having trouble, disconnect the MAF and if it goes away, that is probably it. A crude way but if you want to check, you'd need a scanner to see if it is giving the correct values. It might report 20 units of air when there is only 10 but as long as it is somewhat plausible, I don't think a code will be thrown.

 

When disconnecting the MAF, the backfiring gets really bad and engine will just run very unstable. When reconnecting the MAF it gets better again, so that shows MAF is working.

 

You have a leak there. You have to do it the hard way. Crimp off the hose with hose pliers at the return line, turn the key to run and see if it bleeds off quickly. If it does, get a helper to turn it on, crimp the feed line and see what happens. Report back.

Well, as I said, that is what you say it is based on your light on the timing mark on the harmonic balancer. Howewver, what if the outer ring was moving on you? First off, paint a stripe on the front of the HB all the way from the center out. If EVER you see the line separate, you know you have a problem.

First thing first. Now we have a suspect. With the FPR hose off, it should simulate a WOT condition. I'd have expected over 40 psi. If it doesn't hold, here is the circuit: Fuel pump outlet to the dampner to the fuel injectors to the regulator to the return line back to the tank. Crimp the return line back at the tank and if it doesn't hold fuel, somewhere between the regulator to the pump outlet is leaking. Could be the injectors leaking or the dampner leaking if you still have that on.

If it does hold pressure, I'd suspect the regulator.

 

It shows the MAF is giving some sort of data. Again, I don't know what the values shoud be since I don't have a stock MAF unit but someone should be able to tell you that at a certain air temp and pressure at idle the MAF should read certain numbers.

 

It doesn't hold pressure in either case.

 

Did you mean that you crimped off the return line and it didn't hold pressure? ASSUMING you ahve a good pump, did you replace the dampner with a good hose that was rated submersible in fuel?

 

Yes, with crimping off the return line it didn't hold pressure.
I replaced the fuel pump today as part of the troubleshooting.

Not sure what you mean with the dampner.
But with both the feed and return line crimped off, it still looses pressure.


Looking at this picture from the FSM, it would mean I have a leaking injector. Even if the FPR was shot, since the return line is blocked, the fuel has nowhere to go.
I've checked all injectors and they all read between 16.6 and 16.9 ohm. The cold start injector is at 4.4 ohm.

 

sits between the pump and the metal rail


We need to see if the FPR is sending fuel back via the hose to the intake. Pull hose and see if it smells of fuel. You can get a kit with all the stuff including the right hose to replace your dampner HERE.

If I were you, I'd forget that test altogether. All it tells you is the coil is good. I had an injector off my MB that was good according to the DVOM. You blow through the inlet and air comes out the other end. There is only one test, IMO, that is worth a damn and that is on the flow bench. I test my injectors every 3 years because that seem to be enough crud build up that causes concern. No, snake oils to clean injectors I don't intend to waste money on. The ohm reading gives you a false sense of security that the injectors are good. I can saw off the bottom half and it will still ohm good. If you have Multec injectors, don't bother. Give FIC a call and he will be able to send you a rebuilt set. Also change your FPR since you are there.

 

Yes, there is some smell of fuel at the vacuum hose of the FPR, but it's never wet of fuel.
Just ordered the injectors from that link, thank you.
Have a new FPR on order as well.
Do you have link for the 9th injector as well?

 

There should not be a smell of fuel there. Besides, the rubber is old. Would you use a 20+ year old condom on a fertile girl and hope it doesn't break? 18 minutes of fun gets you 18 years of child support payments.

I would remove that and send it out. Give him a call and have him hold the order till you get your injector out. His turn around time is about a day so soon as your 9th injector gets there, it will come back to you. Same day, that is. Also, he might have one on hand he can charge you for till your "core" gets back. Give him a call. If you are lucky, you might be able to talk to Megan who works there.

 

Zebras that won't cause a backfire. Who cares if the regulator seats perfectly or pump check valve seats perfectly as have nothing to do with operation. The timing can be + or - 10degrees with no backfire it might not run well but a different problem.

Back fire through intake is lean from something.

Air cheating around MAF might be it but easy to find unless the EGR has a small leak, An unlit propane torch finds vacuum leaks fast. Don't forget the connection between MAF and engine or throttle shafts.

From the early posts I think I saw a torn up pump sock in the tank is there trash in the fuel filter or injectors that got past the fuel filter.
Have you checked any of this and a fuel filter test is a new one.
Using data from an O2 sensor might show something looking at cross counts and integrator values but if the problems on the other side of the engine it does not do a thing.

If it backfires through the intake and not a leaking valve from compression test results so cross that off. If one cylinder unplugging one injector at a time will show what cylinder or cylinders as a dead one does not backfire.

It can be a valve open when it should not be or is not open when it should be for a bunch of reasons.

The one thing I have not heard is 'scan tool', it would solve a bunch of mysteries. The other is how old the fuel is and did anyone check for water in it. When you do the filter have a helper cycle the key to pump some fuel into a jar.

These re a start time for turkey soon here. In all honesty 2 hrs or less and 20 bucks.

 

Might show up if the IAC counts are low and cannot be adjusted. Might be a little different for a MAF car but I doubt it. I had an intake manifold gasket leak that was AFTER the sensors and my IAC could not get above 0 counts which made me hunt for the leak.

According to him it won't hold pressure even when he clamps off both return and feed line so the leak is somewhere in the injectors or the FPR area.

Absolutely. Unfortunately most people seem unwilling and/or unable to get one. I checked that the ones at Autozone are too new for our old cars so he is either going to have to beg, borrow or steal an older unit. Maybe a Snap On Brick or get a harness and use one of the free programs to scan.

 

That makes it real simple. If it is 1 or 2 injectors dumping fuel on the same side of the engine as the O2 it explains everything. Pulling the injector connectors would have found just as well. It would kill the rich hole and ecm would add fuel back in. If th regulator was dumping fuel into the plenum most if not all of the cylinder would have gotten some and no lean ones to back fire.

I mentioned pulling injector connects way back when for a good reason it tells a lot in 2 minutes. When I rent engine dyno time I pay 100 an hour to put the engine on and off and 200 an hour when I get there running or not as there are others waiting behind me. You have to divide and conquer quickly or lose $$$$ badly. In 40 years of engine building and tuning I know hoe the cut to the chase quickly.

 

Alright, another update:
- put in new 24lb bosch injectors
- new FPR
- new fuel pump assembly, damper replaced by fuel line.

Fuel pressure is much higher after priming, about 50psi. When running, it drops to about 40psi with vacuum. Without vacuum it remains about 50psi.
When switching the ignition of, it drops steadily to about 30psi.

Unfortunately this still does not solve the problem. The backfiring is still there. As the fuel pressure still dropped, I checked the cold start injector as that was not replaced. It is bone dry, however so also that is not the problem.

 

Weird. That is high. Without vacuum it shouldn't be more than 44, Regulator, maybe?