Start with replacing the filter.

 

Fuel filter replaced recently. Also replaced EGR valve, pcv valve and tube. However, as I understand it, none of these items will cause the fuel pressure to drop once key is off.

At this point, I'm trying to diagnose whether it is injectors/fpr (which I consider a combo job) or a fuel pump, which of course, is located the opposite end of the truck.

 

check the easy ones first.
1. Fuel pressure regulator- pull the vacuum tubing off the fpr and if there is fuel, diaphragm is shot, Replace the fpr. Oh I did not notice you have an old gen 5.7L. FPR is inside the intake. Can't check the diaphragm condition unless intake is off.
2. If fpr is good, injector/s could be leaking or stuck open. but it will cause a rich condition on the exhaust as detected by o2 sensor.
3. Fuel pump built in check valve is stuck open causing loss of fuel pressure during priming. Will cause hard start. Starts but on 2nd or 3rd try.

 

Thanks for the response!! The pressure spikes to normal range (56-61)when first turn key on to prime, then loses pressure quickly (before starting truck). Can this only be caused by the check valve?? Otherwise, it starts pretty easily when engine cold if you turn key to prime 2-3 times first, but runs very rough at idle. Holds pressure at 52psi (normal is 51-56). Then drops after you turn off.

I am getting a P1406 code, which according to Autozone is likely injectors or intake manifold gasket. I see a small intake manifold leak on driver's front corner, so I do have that, but not sure if that is related to fuel pressure.

Unfortunately, the fpr is located underneath plenum, so cannot get to it.

 

I made a video of the fuel pressure test, if it helps.

 

Is it hard starting without priming? If yes, bad fpr, injector leaky and fuel pump check valve could be culprit.

P1406 is for the EGR valve. Different issue from fuel pressure drop. EGR or passages could be blocked by carbon buildup.

Intake leak, injector and fpr leak can cause P0300 multiple misfire though. Idle will be unstable and rough.

Injectors and fpr can be checked with upper intake off with fuel lines connected and fuel pump primed.

 

Yes, must prime before starting to avoid hard start. I replaced EGR valve and pcv valve/tube when first got code. No change. Also have the P0300 misfire code.. Idle is exactly as you describe.

Am soaking EGR tube in pb blast now as it is coated in rust. That is my main concern to removing plenum to check. Also figure if I'm going to remove to check then may as well replace the injectors, fpr and intake manifold gasket.

Can I pinch the rubber hose part of return line by fuel filter to see if it is pump?

 

Someone is confused here, I’m sure it is me. You are working on the 7.4L Vortec, correct? Someone said you were old gen 5.7. Pulling the plenum isn’t too difficult to do some testing on the fpr. I don’t think a bad check valve in the pump is going to give you rough idle. Pressure looks consistent while running. Multiple injector failure is also unlikely, but with 200+ on the odometer the injectors are likely tired so multiple leaking is a possibility. You certainly won’t hurt anything changing all the injectors and the fpr. It’s on my to do list.

 

I so appreciate the replies and watching my video!! Thanks for reassurance that it is more likely fpr or inectors than the pump. Looks like the only hard part to removing plenum is disconnecting EGR tube, but I'm MUCH more comfortable working on things like suspension/brakes/drivetrain than under the hood.

I used to own a 1993 Suburban K2500, 5.7L, which is the vehicle I first learned (the very hard way) to wrench on. Now I own a 1996 Suburban K2500, 7.4L, and it is quite different to me. It was in really nice condition when I got it 2 years ago, so I've only done some super basic work on it.

I've had it parked for months, while reconstructing a 1992 Jeep Wrangler YJ. But with winter setting in, time to get my burb back into tip-top shape!!

 

Shopping for parts to buy today. On my list I have Delphi FPR, Fel-pro gasket, AC Delco plugs, wires, cap, rotor (figure I may as well do a tune up while I'm at it). The big question is for the injectors.

It looks like Bosch 4hole is the way to go for injectors. A brand new set is $400, but a refurbished set is under $100. Any reason I should not get the refurbished set??

 

I have enough other things to spend money on so I would go with the rebuilt. Give you $300 to spend on something like LED lights!

 

Parts ordered, and went with the refurb injectors. Agree that extra money is better spent on other things for the burb. Everything is scheduled to arrive by Friday. I'm continuing to pb blast EGR tube daily and am almost done giving the engine bay a thorough cleaning, which helps me to learn parts and assess other possible issues. Will probably start removal process tomorrow.

 

Started tearing into burb yesterday. Got everything removed except the stupid EGR tube. Found gas stain on passenger side of plenum, which I believe is evidence of leaking injector. Parts arrive today. If we can get the egr tube removed, we should be able to get it all put back together by tomorrow.

 

UPDATE: We got the intake manifold off yesterday and found pool of gas laying on bottom. Also discovered FPR vent hose soaked in gas. Pretty sure we found our problems. We stalled on removing fuel injectors when we realized we had to remove fuel rail, so will tackle that first thing today. All removed parts have been given a thorough cleaning and are ready to re-install.

 

This project is turning into a royal pita. Had to make trip into the city for TBI gasket, FPR hose, and a special tool to get FPR spring clip on, which took half a day. But finally got manifold back on before calling it a night last night. Trying to motivate myself now to go outside and finish putting burb back together to test fuel pressure while hubby at work.

Of course, we can never have a repair project with the burb that doesn't result in breaking parts along the way, and this job is no exception. My husband accidentally stepped onto the mess of heater hoses and broke a t-junction piece when putting manifold on. So now we have to figure out how to fix that. The fun continues..........

 

Well I almost finished putting burb together today while hubby at work. I still need to fix a broken ground located on t-stat housing, then mount AC compressor, put serp belt on, and install/connect air filter and battery. So in the morning we get to finish up and test fuel pressure, then back to the store we go to return fuel pressure testing kit and get more parts. Pretty sure the piece that is broken is called the heater control valve. Hoses connected to it are pretty beat up, too.

 

Massive failure on test yesterday.
After running back to store for heater control valve and replacing that set up, we tested the burb. While it fired right up and idled strong, we found an obvious leak coming from rear of intake manifold towards the passenger side. Also noticing smoke coming from passenger side middle spark plugs, near where the heater hose valve exploded coolant. While I'm wishful thinking the smoke is from splashed coolant, pretty sure those injectors are not installed correctly as the fuel line connections, fpr and gasket seal all appear dry. Guess we get to do this fun project all over again today.

 

Did you use Felpro gaskets on the intake? And used a torque wrench? And does your 7.4 use an aluminum or composite intake manifold?

Ted

 

Yes, it is a felpro brand gasket. I used torque wrench (although not the best torque wrench for 100% accuracy, it hasn't let me down before), and tightened bolts in order per book spec. I did brush a teeny dab of gasket maker on outside ends to help hold it in place when lowering plenum onto it. I believe the plenum is aluminum, but not 100% on that? It definitely is original stock. Not seeing any obvious signs that it is leaking from there, unless leak on inner edge, as the fuel is coming out from behind the manifold and gravity is pulling it just below distributor cap and off towards passenger direction. I'm hoping that if I turn burb on for a minute before tearing it apart, the leak/fluid will be fresh and I can better identify source as soon as lid is off.

If I had to guess I'm thinking we did not install injectors correctly, and I am concerned they are not fully seated/sealed. Also, we broke half of the nubs on the rail removing old injectors, so did not put clips back on injectors, which may also have been a big mistake. This time around, we will put clips on and attached clips to rail with zip ties.

 

Felpro are good quality gaskets. They should not have been a problem if you had clean mating surfaces. It's good to read you torqued the intake correctly. I asked if the intake was composite because that material could have cracked if torqued down too much.

It would take quite a leak to leak fuel from the manifold. When manifolds leak it's usually a vacuum leak to one or more cylinders. From what you say there is fuel under the intake manifold. That injector should fire when there is vacuum in that runner so it's unusual if the runner is leaking fuel at the head. I think it might be leaking at the o-ring seal at the fuel rail. Or it's possible the fuel rail itself is leaking.

If you have a scan gauge and you're going to start it before tearing it down again pull the codes and get short term and long term fuel trims for that bank. Cylinders 2, 4, 6 and 8. That might flag a vacuum leak. I'm thinking your leak may be outside the manifold.

Ted

 

Ted,
Thanks for helping me try to figure out where things went wrong and how to get it fixed correctly! While I usually get the cheapest ebay part for my Accord, I try to get the best quality for my burb.

I cleaned mating surfaces really well with carb cleaner. I also cleaned every part that we removed, and replaced/repaired grounds, gaskets and vaccuum/vent lines as we went along. Gasket seal looks good along outer and front edges of manifold. We took a lot of care to set it in properly, given my previous nightmare replacing the intake manifold gasket on my old ‘93 5.7L burb.

I am thinking the same exact thing as you, that it is either leaking at rail (and we screwed up by not using locking tabs) or rail itself is leaking. We couldn’t remove bracket on return fuel line to remove fuel rail, so it took some maneuvering of rail to get injectors into place. Wouldn’t surprise me one bit if we damaged rail in the process.

I do not have a code puller. I have a multi-meter, if that is of any help?
Janet

 

We got it all apart and found the leaking injector. Iain wants to just try to seat it better by pushing on it. I'm insisting we remove, clean out, make sure o-ring is on correctly, and re-install. But in order to do that, you have to remove all injectors from manifold to remove the one injector. So we are at a temporary stand-still.

 

I like bizar01...check the easy ones first. never dive in till you know how deep the water is. luck to ya.

 

Sounds like y'all have it covered. Next I want to see that it's all squared away.

Ted

 

We have finished re-seating injectors and fuel rail re-installed. The driver's side rear injector was completely wet with fuel from it leaking from fuel rail connector, which would explain why I was seeing fuel so quickly just behind intake.

Now we just have to put it back together once more........

I assure you, one way or another, maybe tonight or next month, there will be a happy ending to this thread.