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Below normal fuel pressure- backfiring under throttle

B

Blue00GTcoupe

New member
Hello,

I have been working on diagnosing this problem now for 6 months, fortunately the car keeps running.
I have had misfire code on cylinder #3, now I suspect after checking for intake leaks, vacuum leaks and finding none of either one, that its a lean fuel condition. The car runs fine as long as I don't push the throttle down more than 1/4 of the way. Past 1/3 down and it starts stumbling and 1/2 way down or more I get the late ignition backfire into the intake, which I think is a lean issue.

I tested the fuel pressure, and the book says that "normal" is 48 to 55psi with KOEO test on a NA 3.8 L, mine was 45 at best.
Idling, mine was at 39 psi, giving it a good rev in park, it maxed out at 49 psi. It did jump up on the pressure nice and quick, I will say that.

So how do I narrow down the problem without changing a bunch of un necessary parts? :th_nervous:

Thanks for the help, for any yahoo's out there who reply.


BTW: I changed the filter on the tranny at 135k miles over the winter and filled with Amsoils ATF, and found the old filter was totally blocked. When I went to pull it, not a drop was coming out, then a huge gusher once it was off and out. Probably a good idea to change that filter, don't just do the reverse flow jiffy lube method.
 


The fuel pressure you're reading sounds pretty typical. You may try changing your fuel filter. Maybe you have a partially clogged injector? You check the #3 plug to see what it looks like?
 
Your right, a backfire thought the intake is a lean issue, so you abviously have a fuel delivery problem, could it be a clogged fuel filter?
What you need to do it hook up a fuel pressure gauge and position it so you can watch it while you drivre the car. This way you can watch and see what the pressure is doing with the engine under heavy load. Chances are you will ssee the fuel pressure drop very rapidly, the pressure will slowly, or not at all recover. This is what causes your backfire through the intake.
 
Okay, so I need to verify with a well positioned pressure gauge. %&*@# ! My fuel pressure testing kit has a 11" hose, so I guess besides cutting a hole in the hood or taking it off altogether, I don't have the equipment.
 


What I did was use the copper 1/8" tubbing used for oil prressure gauges, and adapted it betwwen the hose and gauge so I had enouff length to lean it on the windshield for visiblility. Now, you are not looking to monitor the exact amount of fuel pressure, but rather the fact the fuel pressure falls under load and that pressure does not stay the constant that it should,so visibility of the needle only needs to be such that you can see it move.
 
Has anyone out there had one bad injector that was only noticable under 1/3 to full throttle, where it was clogged?
What is the size and type of injector used in the 2000 NA GT?

Any one have a good one handy?
 
I changed the fuel filter, and there was a slight difference for the better. I bought some adaptors so I can see if the fuel is dropping while driving.

I keep thinking that as a hunch, maybe the in-tank fuel pump mesh screen is clogged with debris. The P.O. was putting on used and warped tires on this car, so why not siphoned or drained gas from other machines? :th_skull: Thank God he didn't change many other parts. :th_nervous:
 


I have not changed the injector position, but I will try that soon and see what happens. The code now says random misfire, instead of previous misfire code cyl #3.



What Lb/ hr size are these injectors? I would assume they are high impedance?
 
I'd check the EGR tube from the LIM to the EGR. It's a common vacuum leak that can only really be diagnosed off the car.
 
The fuel pressure has been tested while driven now, the results are 39 psi at light cruising, and 49 psi at half to full throttle. Seems normal but maybe I am wrong. Would surging under throttle be a symptom to look at too? In the past, surging on other cars was associated with Mass airflow sensors sometimes.
 
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Just bought a new set of new flow match tested 4-hole gen II Bosch 19 lb. injectors, high impedance for my G.P.. I also cleaned the MAF and TB this morning and tested it, made alittle difference but it's not fixed yet. Well see how the injectors work, I figure with almost 140k on the clock, original ones in there, probably their crapping out one by one.
 
Uh..you changed injectors w/o changing the table in the pcm? And 19 lbs would be very small for our setup. We have uih..36's stock IIRC.
 


Remember, this is a Normally Aspirated V6, not a SC.

My 97 silverado 454 has 19 lb/hr in it stock, which puts out 290 hp, so I figure if I am running at 200 hp stock on 6 cyls, then the math goes like this:

290hp ////200hp
152 ////////x

152 is the collective lbs. per hour x 8 cylinders on a stock 454, so X= 104 divided by 6 cylinders for the Grand Prix = 17.47 lbs/hr per injector. The stock ECU can adjust up or down up to 25% for fuel trims, so I think I will be in the proper range.

Also these new injectors flow at 19 lbs/hr at 42psi and 21 lbs./hr at 50 psi.
 
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