So here's the scenario. Getting car back on the road after sitting for two years (new baby, job, etc.). Fires right up but idles low and distinct backfires as it revs up, unpredictably. Was idling at 14.7 on the AF gauge, until I chased down a couple of vacuum leaks. Now it idles much leaner, like 17 or more. Still has distinct back firing as it revs and as I let off the throttle. As I rev it up from 3500 to 4000 it is at 15.5-16 on the AF gauge.

Here's what I did as it sat: new LR injector harness, bosche 3 bar fpr, rebuilt injectors (witch hazel), new fuel lines, ran an AF gauge and boost gauge, deleted CV valve and ran MBC to a LR dual port wastegate, evergreen rebuilt turbo 26/6, rebuilt alternator, rebuilt head with mild P&P, new head gasket, valves, seats, etc.

Where should I start?

 

Check all your vac lines, make sure you have them on correctly and routed correctly. Sounds like a large vacuum leak

 

Check to make sure all your cylinders are getting fuel, Pull plugs one by one and see if it changes or stalls. That will make it pop and also show a lean afr. Possibly one of the new harnesses has a bad cnection

 

I'll try the plugs tomorrow. I pressurized the system today and fixed all vacuum leaks. It actually started to run leaner after I found the two leaks.

Two other pieces of info: new ISV and rebuilt the throttle body while I had it off.

Was running fine, except for a bad alternator and a turbo blowing oil, prior to the head work, etc.

Why would a bad plug cause a lean condition? Thought it would throw unburned gas down the track.

 

It won't, Pulling plug wires one at a time and listening for a change is an easy way to see if an injector is not firing. If you have an injector that isn't working properly it will cause popping and a lean afr. I'm not saying it's the problem but I would def start there. If you pull a plug wire and there is no change it he idle you have found an injector that's not working. If you pull a plug wire and it stalls, that is not the one but that also means you DO have a cylinder not firing, it's just one of the other 3

 

If that's too confusing just pull the injector clips one at a time and run the engine. If it doesn't change the idle then that injector isn't working properly, I normally do the plug wires bc they are quicker

 

Nice suggestion. Thank you.

 

I would start by pressure testing the intake track as shown here:

 

Did that today. Found a few small leaks. Fixed same and it is running leaner now.

 

Are you running AFM and stock chips? I would probably check the O2 sensor next. Closed loop should get you 14.7 unless your tune is way off. Any difference with O2 sensor unplugged? Also, check wiring to the DME temp sensor.

 

If it's backfiring while it revs up, I'd quad-triple check that the timing belt is correctly aligned. If that checks out, then look for vacuum leaks as suggested and measure the amount of vacuum at idle. Your car might have become leaner after fixing a few leaks because the FPR was not getting a strong vacuum signal and was delivering more fuel pressure than needed. Why the 3 bar FPR -- what chips/afm-MAF/injectors are you using and are they mapped for 3 bar of fuel pressure? Spark plug wires in the right order? AOS and ISV hoses mixed up? Intake fully seated on head? Any of the intake gaskets swing out and miss a bolt on install? Sure you installed the LR injector harness properly? Did you plug the return port on the J-boot when you deleted the cycling valve?

 

Nice list of things to check.

I will check timing again. Can't hurt. What are the symptoms of timing being off by a notch?

Stock AFM and chips for now. Was planning on going to something more custom shortly.

Got the Aos and ISV correctly connected.

Pressure tested the intake so I doubt there is a skewed intake gasket, but that's easy to check.

I can easily put the 2.5 bar fpr back on to see, but wouldn't the 3.0 more likely cause a rich running condition?

I'll check the o2 sensor and throw it into closed loop.

I like having a list of things to check.

 

So far I have pressure tested the intake and isolated/fixed a few vacuum leaks.

Confirmed cam timing three ways to Sunday.

Going to pull the 3 bar fpr and put the 2.5 back in and confirm injectors are functioning properly next.

 

Are you backfiring out of the exhaust or the intake? Backfiring out of the intake as you rev the engine usually indicates a lean condition, backfiring out of the exhaust when you release the throttle indicates unburnt fuel or retarded ignition timing. If you backfire excessively when you downshift and coast to a stop light, id check your TPS to make sure its giving a good idle signal to the DME.

 

After confirming that I am the world's biggest idiot, I discovered I had the firing order wrong, and she is running fine right now. I have to shake her down and now hope that the leaks which were caused by sitting too long will work themselves out.

Can't thank everyone on this forum who contributed to finding the source of my problem enough.