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I have been working on 1998 chev suburban 454 engine has

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miss, new distributor, coil plugs...
I have been working on 1998 chev suburban 454 engine has miss, new distributor, coil plugs wires, egr tps map sensor, when replaced coil seem to get worse when you start when cold it stumbles and then after about 30 seconds smooth's out, when warm it misses slightly at idle under power seems to hold back, then when you shut off and restart it just cranks unless you let up on key a little the starts, thought this might be ignition switch so I replaced but is the same. no codes ??????
Submitted: 2 years ago.Category: Chevy
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Answered in 5 minutes by:
8/30/2015
Chevy Mechanic: Pete, Auto Service Technician replied 2 years ago
Pete
Pete, Auto Service Technician
Category: Chevy
Satisfied Customers: 30,484
Experience: 14 years GM dealer Technician.
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Hi my name is ***** ***** I will be happy to assist with this here.

Most common on this model that I have run into several times on the 454 (7.4 liter) engine is leaking fuel injectors under the intake upper plenum.They drizzle fuel down into the intake with the key off causing hard starting and then when started rough running for a but until it leans out and smooths out.In some cases when it will not start you can tell this is the cause here by holding your foot on the gas pedal to the floor while cranking over the engine to see if this clears up and allows starting.If it does the best way to get a quick check that this is the cause is to put a fuel pressure test gauge on the test port and turn on the key.The pressure should rise on the gauge and hold and hold for several minutes.If it starts to drop pressure this indicates one of the injectors is leaking down.Then remove the upper intake plenum to where you can see the injectors and turn on the key (without cranking over the engine) and see which injectors are leaking down fuel as these would be the ones needing replaced.

I've replaced lots of the injectors on this engine for this exact fault here.

Thanks Pete

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Customer reply replied 2 years ago
Pete I am sorry it is 1995 tbi sorry .
Chevy Mechanic: Pete, Auto Service Technician replied 2 years ago

No problem,has fuel pressure been tested at all?And if so what was the pressure and does the pressure hold for a few minutes or does it drop off?

Let me know,thanks Pete

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Customer reply replied 2 years ago
no it has not no test port on those models is supposed to be 8-12 psi can I test be for filter, filter is new
Chevy Mechanic: Pete, Auto Service Technician replied 2 years ago

O.k.,there is an adapter that threads in where the fuel filter is located to where a pressure test gauge can be installed.In some cases you can rent this adapter and fuel pressure test gauge from a parts store to test this.I would see if you can do this testing as two of the more common issues next to what you have done already are a leaking fuel pressure regulator allowing to much fuel back to the fuel tank or a weak fuel pump needing replaced.If the pressure does drop off after you prime the pump clamp off the rubber section of the fuel feed line leading to the engine and see if the pressure holds then.If it does this indicates the fuel pump is faulty letting fuel drain back through its check valve and is needing replaced.If it continues to drop off this indicates the fuel pressure regulator is faulty needing replaced.

Thanks Pete

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