I don't know if GM has ever published "bleed down" specs for these engines. Ford (which used a similar recirculating system in the early to mid '90's) published the bleed down spec as something like 1 psi in 10 minutes. Basically, if you can look at the gauge and visibly see it dropping, it is probably bleeding off faster than desired.

I would not automatically assume that it is bleeding off through the injectors. If I remember correctly, your 7.4L has an "external" fuel pressure regulator. One common failure mode that I have seen with that kind of regulator is for the diaphragm to develop a leak and allow the excess fuel to bleed into the intake through the vacuum line attached to the FPR. Run the engine, then disconnect the vacuum line going to the FPR. If there is gas in that line, then the FPR needs to be replaced.

 

Thanks for the suggestion on checking the vacuum line to the fuel pressure regulator. I see the FPR mentioned in the shop manual along with a description of replacing the FPR. Unfortunately, I have not positively located the FPR on my vehicle. I see a glimpse of something that may be the FPR at the back of the intake manifold, near the distributor. I can't quite get my hand on it to determine if the item I can barely see is the FPR and whether it has a vacuum line attached. I see what appears to be the solid fuel line on one end of this thing but I have to say it's difficult to see back there even with a flashlight.

 

But, if it bleeds down enough to cause starting problems when hot, it should be completely without pressure when cold. Why would it start easy when cold?

 

With this type of system, the reason it mostly or almost exclusively effects warm starts is like this:

1) On cold start, the intake is free of gasoline. The only (significant) source of gas is the injectors.
2) When the engine shuts off, excess gasoline leaks through the FPR into the vacuum line and intake.
3) If you come back before this excess gasoline has time to dissipate out of the intake, it can be hard or impossible to start because there is too much gasoline in the intake -- essentially the engine is flooded. If it does start, the engine will tend to run really rich until the excess gasoline has time to burn off.
4) If you wait long enough, the excess gasoline will dissipate from the intake, and the engine will start normally.

 

Ok, I'll buy that. Thank yoj

 

I'm planning to buy a simple fuel pressure gage this week and I'll do some more troubleshooting. I'll report back and let you know what I find.

I'm leaning toward replacing the fuel injectors and the fuel pressure regulator even if I don't specifically find leaks. At 160,000 miles on the odometer it seems like this would be a good idea.

Some of the best prices I'm finding are Delphi brand parts. I recognize the name Delphi from GM electronics such as radios and similar products. Is this the same Delphi? Are the fuel injectors and pressure regulator from Delphi good quality? The ratings I see for Delphi appear good but there are some less than 5-stars. Comments welcome.

 

I purchased an inexpensive fuel pressure gage at Harbor Freight. The gage leaves a little to be desired in the feature category - for instance it does not have a bleed down button/knob to release the pressure after testing. However, the price was right at $20 retail and I had a 25% off coupon.

After bleeding the pressure I put the gage on the nipple and turned the key on to let the pump run until it turned itself off after a few seconds. With the first on-off sequence the pressure came up to 30 psi or so - somewhat lower than spec of 56 - 62 psi. The second on off cycle brought the pressure up to just under 60 so right on target. The third on off cycle didn't change a bit and it stayed at 60.

Then I started the engine and let it run for 2 minutes of so. The pressure was very steady at 50psi. A little lower than spec but seems good that it's steady. When I turned off the engine the pressure remained steady at 50 for about two minutes then started dropping.... 45, 40, 35, 30 down to 25 psi after 30 minutes. So something is leaking. Now I hope to determine if it's the fuel pressure regulator or the injectors.

I found some pictures of the fuel rail and see the fuel pressure regulator is located at the base/end of the rail where the fuel line enters the rail. On my vehicle - 7.4L - that location is right about at the top of the bell-housing but it's sandwiched between the firewall and tucked up under the upper half of the intake manifold. I can barely get my hand on it and I can't reach the vacuum hose that's connected to the fuel pressure regulator to see if it's leaking gas into the vacuum line.

If you have any comments please chime in. Thanks.

 

I'm having a similar problem with my '99 7.4 Suburban: starts fine cold, but a bit hard to start when warm. If I give her some gas when I'm cranking on a warm start she gets going just fine. Does this work for you?

After searching the intertubes I concluded that the FPR was the likely culprit, but you do need to remove the upper intake to get to it.

Anyway, I'm curious to see what you figure out nve.

-Alex

 

I'm pretty sure the 7.4 is like the 5.7 and the FPR is under the upper intake toward the rear. They are pretty cheap and relatively easy to change. As far as the injectors go, there are not individual injectors. Instead, they are all a part of the spider looking thing in the same area as the FPR, under the upper intake. There is an updated version of the injectors out now that is better than the original design, but my bet on yours and Alex's problem is on the FPR. I have done several of the FPRs on 5.7's and 4.3's and it only took a few hours.

 

The 7.4 is SFI (sequential fuel injection). it has individual injectors mounted on the fuel rail under the upper intake manifold. no "spider" like the 4.3's. and yes you need to remove upper intake to replace the fpr.

So Gary, you'd say removing the upper intake (and putting it back) isn't that tough of a job? I also need to redo my valve cover gaskets, but I'm a little nervous about taking so much apart and I've heard one can mess up the timing attempting this job.

Not meaning to hijack the thread here nve...

You don't have to worry about your timing unless you remove the lower intake manifold. this would require removing the distributor.

 

So Gary, you'd say removing the upper intake (and putting it back) isn't that tough of a job? I also need to redo my valve cover gaskets, but I'm a little nervous about taking so much apart and I've heard one can mess up the timing attempting this job.

Not meaning to hijack the thread here nve...

 

Thanks for the clarification Ed. I have not worked on the fuel injected 454's, so I am not familiar with the injection setup. The 454 in my Big 10 is old school carbed so I haven't had the pleasure of tearing into a newer one yet. With all that said, I would still put my money on the FPR.

 

I still haven't done anything to solve my problem yet. After catching up on this thread I see I'll need to take off the upper intake to access the FPR. That makes sense as I can't see any way to get to it otherwise. A previous post indicates a few hours - I can believe that. Some other threads indicated the FPR could be changed in a matter or minutes but that must be on other engines.

I ultimately need to remove my lower intake manifold too because the gaskets are leaking at the intake/head and I'm loosing a small amount of cooling fluid. I guess I'll tackle the entire upper/lower/FPR project when I get a few warm days.

For now, let's assume replacing the FPR fixes the low fuel pressure reading and the pressure bleed-down. After that, if it still takes a long time to start I can presume it's the fuel injectors. It appears I can remove and replace fuel injectors with just the upper intake removed. Is that correct?

I know I have to drain coolant to remove the lower intake. Do I need to drain coolant to remove the upper intake?

 

If the FPR fixes the fuel pressure issues, it will probably fix your hard start. i would recommend replacing the FPR and rerunning your pressure test before re installing the plenum. with the plenum removed, the fuel rail and injectors should be easy to remove or test if need be.

There should be no need to drain coolant to remove the plenum.

 

I decided to change the fuel pressure regulator, all the injectors, and the fuel rail. I originally was not planning to change the fuel rail but several retainer clip "nubs" broke when I was removing the clips.

My lower intake manifold gaskets were leaking so that was part of this project too.

The vehicle is running and seems to work fine. I'll be driving it exclusively for the next several days to ensure it's working under a variety of conditions. The engine stared quickly when cold. It started quickly when stopped for just a few minutes, and it started quickly when stopped for an hour. Starting after an hour was my main problem.

So now I'm going to check it under real world driving and will post more details in a few days.

 

So far so good. The Suburban starts quickly under all conditions in the past few days. I've only driven 100 miles or so but there have been quite a few short distance trips so I've had several occasions to turn off the engine, let it cool down to varying degrees from 10 minutes to 1 hour, to 2 - 3 hours. In each case the engine has started quickly. It's really nice that I don't have to hear that 15 second or longer cranking cycle that I had become so accustomed to.

 

Great update. How difficult was it to replace the FPR on your 454 7.4L engine. Not sure it is the same as the one in my 1999, but I'm sure it is similar. This is something I will have to do at some point. As well as a full maintenance service - plugs, wires, etc.

 

I looked at the online repair guide from my local library and your 1999 7.4L appears to the same as my 1996. The FPR is buried under several items near the firewall so it's not the simple 20 - 30 minute job that others mention for their engines.

In retrospect (hindsight is 20:20) I probably should have replaced the FPR first to see if that fixed my starting problem. Replacing the FPR requires removal of the upper intake manifold and associated wires, connectors, hoses, EGR and a handful of other items I'm forgetting about right now. Once the upper intake is removed you can remove the spark plug wires from the distributor cap, remove the distributor cap and you may need to remove the distributor too for easier clearance. I could probably do all that in four hours at this point but it took me about eight hours the first time around. Reassembly went much more quickly.

I also had a leaking gasket at the interface of the lower intake manifold and the heads. So I needed to remove the fuel rail so I could get to the last bolt securing the lower intake. All that work so I just decided to replace the injectors too. While removing the fuel rail I broke some of the injector clip retainer tabs so that lead me to replace the fuel rail.

So I'll never know if my starting problem was simply the FPR or something more. For about $600 or so I've solved my starting problem and leaking gasket problem.

Here's a picture of the fuel rail from the online manual (same picture and descriptions as my GM manual). The FPR is at the rear of the rail.

When I started the project last week I joked to my wife that there were only 15 steps listed in the manual to do everything I wanted to accomplish. Step one: remove the upper intake manifold.... ha-ha!

 

All is well after several thousand miles and several states - crossing state lines that is. We took a trip to Texas and the Suburban has been starting flawlessly. I wish I had done this sooner. We went for years with the vehicle slow to start (long start of about 10 seconds or so) when the engine was warm - not hot, not cold.

As I mentioned above the fuel pressure regulator was likely the culprit but who knows for sure since I went ahead a replaced the fuel injectors too. Since the problem has been going on for several years - back when the vehicle had much less than 100k miles - I now suspect it was the FPR.

By the way, the gas mileage has not changed.

 

Chevy liked to run the fuel line along the engine and on big blocks,a common problem was bubbles inside fuel line causing vapor lock.fuel bubbles so u don't get constant liquid. I don't know if it looses power after operating temp.if it does,reroute fuel line.some drag car owners coil fuel line inside a coffee can under hood with ice before it goes to carb.just trivia. Steel lines running along the block too far cause vapor lock.good luck.