I've been trying to diagnose why my 1996 GMC Suburban 7.4L is hard to start when the engine is warm. By hard to start I mean it takes a long time to start 20 - 30 seconds. The engine cranks over fine so there doesn't seem to be a problem with the battery or starter. When the engine is cold it starts quickly - just a second or two at most. But after the engine has been run for a while and I try to restart the engine it's very slow to start and I get a "rich" smell of unburned fuel from the exhaust pipe.


So that's the story - now on to what I think may be the problem. Are my fuel injectors leaking down? I've been researching this but not quite sure how to diagnose the problem.


I see a pressure port for the fuel system at the front of the engine. What device do I use to check the pressure? I'm guessing I don't use a simple pressure gage like I use to check my tires. Stop laughing... I'm relatively handy with tools but not familiar with all the special tools you might have in your garage.


When I check the pressure I see other threads that indicate a pressure range for the fuel system. What's the correct range for my 1996 7.4L? How long should it hold that pressure when I turn off the engine? If the pressure drops quickly I guess that's an indication of a leaking injector(s). How do I determine which one is leaking? Maybe it's more than one.


On a related note - the local Chevrolet dealer wants well over $1000 to replace the upper and lower intake manifold gaskets. Instead, I'll do this myself and that's the perfect time to diagnose and replace fuel injectors. With 160,000 miles should I just go ahead and replace them all? That's expensive but I don't want to do this twice.


Which injector brands are best? I like the idea of AC/Delco but am open for suggestions.


Thanks for your assistance.

 

I was able to find a shop manual for the 7.4L and found the fuel rail pressure should be 56 - 62 psi. Now I'll have to obtain a fuel pressure gage to check the pressure and see if it meets specs and if the pressure bleeds off quickly.


The shop manual describes using a fuel injector tester to determine if individual injectors are working properly. Based on the pictures of this equipment that looks like a factory/dealer instrument. Will local shops (not dealers) have this type of equipment?


I'll keep looking for answers here and elsewhere. If you have any suggestions I'd like to hear from you.

 

Harbor Freight sells a fuel pressure gauge cheap.

Sounds like one or more injectors is leaking down.

 

I purchased the inexpensive gage at Harbor Freight. Thanks for the tip. The gage leaves a little to be desired in the feature category - for instance it does not have a bleed down button/**** 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 and then the pumps turns itself off after a few seconds. With the first key 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.


I think something is leaking. Now I hope to determine if it's the fuel pressure regulator or the injectors.


If you have any comments please chime in. Thanks.

 

That bleed down could also be the fuel pump...
That initial reading of 30psi concerns me enough to think the fuel pump is the problem to the hard starting...but it could also be a part of the whole issue...
The regulator is a good place to start... cheaper and easier too...