Chev 1500 intermittent starts

DCM · Jan 5, 2011

Neighbors 91 chevy 1/2 ton just cannot be trusted, at times it will start and run like new and other times will not run but turn over. Could this be a computer issue, timing or whatever, any thoughts?

JDemaris · Jan 5, 2011

M first suspicion would be the fuel pump sometimes not making enough PSI to open the regulator. That causes a either "run perfect" or "no start" condition - off an on. True with TBI, and even more true with the early "tarantual" higher-pressure multi-port system.

Revredneck · Jan 5, 2011

Could be fuel or ignition. The easiest way to tell for sure is to check it for spark at those times when it won't start, then you'll know where to start checking things out. I've seen faulty distributor caps and intermittent ignition modules cause that kind of problem, as well as fuel pump/filter problems. Another thing to check is the ground wires on top of the intake manifold, especially the one on the thermostat housing bolt, if it hasn't already been moved to a better spot. I've fixed lots of them by moving or even tightening those ground connections. HTH
Good Luck and God Bless.

Anonymous-0 · Jan 5, 2011

My 94 Chevy w/6.5L Turbo diesel is the same way. Can't take it any place because you never know when it is going to start or quit. It's frustrating.

El Toro · Jan 5, 2011

The ECM may be the problem. Are you getting any check engine lites? Hal

Allan in NE · Jan 5, 2011

Those ol' gals had two weak points and not necessarily in any order:

Fuel pumps and ignition cap/rotor/vent corrosion. The corrosion more often than not.

Allan

Jim In NY · Jan 5, 2011

JDM: I'm with you. First check would be the fuel pump. Sometimes when it won't start an individual can get the pump running by rapping on the gas tank and jarring the pump enough to get out of a must tow spot. A good check for a bad pump also !

blinwmiaol.com · Jan 5, 2011

I have a 96 Sierra doing the same exact thing right now. Been at the mechanics for over two weeks and he can find nothing wrong with it. 95% of the time it starts fine but once in a while it will turn over till the battery is dead but never fire. It did it one day at the restaurant right next door to the mechanic and he got on it right away, tested fuel pressure at 54 psi, tested spark, tested ac voltage from the pickup coil, everything tested fine. Left the truck set for 2 hours and it started up almost before you got the key in the ignition. We just cant find anything wrong, it has fuel, it has ignition and the timing in on but when it decides not to start, it just won't. Any ideas on this one?

Inno · Jan 5, 2011

Fuel filter? Had the one in my '98 clog up.......it would start but wouldn't make much power.

JDemaris · Jan 5, 2011

When you get a reading of 54 PSI, that's the point where sometimes it will start, and sometimes it will not. Especially a Vortec 95 and 96. Takes 55-60 PSI to open the fuel-pressure reguator. Just a few PSI below, and it will not start. Very common problem in 95 and 96 GMs with the CPI injection system (Not TPI).

My 95 Astrovan with the 4.3 Vortec would not start at 55 PSI gauge. At 58 PSI, it started and ran perfect. Central Port Injection (AKA Tarantula).

Note than when you're talking just a 1-2 PSI making the difference, it's hard to know when that gauge is calibrated perfectly. That's wny many repair shops replace the CPI unit and fuel pump when repairing.

Brian Jasper co. Ia · Jan 5, 2011

A 96 Sierra/Silverado will not be a TBI system, it's sequential port. They run 35 psi under vacuum. If you opened the intake to get at the regulator and plugged it, the pressure should jump to 45. TBI systems run 15 psi.

Brian Jasper co. Ia · Jan 5, 2011

One thing to check is the coolant temp sensor near the thermostat. I recently ran across a truck about the same age that someone had screwed up the connector on the ECT sensor. When it shorted together it signaled the PCM that the coolant temp was -40. It responded by dumping excess fuel into the engine and flooding it. You might smell the oil and see if it smells like gas. Otherwise, it's either missing spark, fuel, or both. Got to see what's missing if they're no DTC's to go by.

JDemaris · Jan 5, 2011

I didn't say "TBI", I said "CPI" , i.e. central port injection that was common in 1995 Vortec engines. In 1996, CPI got updated to CFSI.

On either when the regulator gets worn, it can sometimes take 85-80 PSI to open it and the engine start.

Bob · Jan 5, 2011

Actually, when the fuel pressure regulator opens it simply bypasses fuel back to the tank. The engine will start and run if the regulator were to stick and NEVER open but the fuel pressure would be HIGH and it would likely overfuel beyond the computer's "fuel trim" range of control.

The REAL cause of a "no-start" with that CPFI system is that there are spring loaded poppet valves at the "nozzle" end of the tubing from each injector and those need a certain amount of fuel pressure to open and flow fuel, much like the "cracking pressure" of a diesel injector.

<img src = "http://i56.tinypic.com/25zrz35.jpg">

JDemaris · Jan 5, 2011

To be technical, they ALL regulate the flow of fuel. One poppet per cylinder and a central, non-replaceable fuel regulator for CPI. The entire unit considered non-servicable. The CFSI that followed is.
Each poppet has a check valve and spring, set to open at a preset pressure and certainly IS a regulator. Not not referred with the phrase "fuel pressure regulator."

With CPI, the poppet regulators need around 40 PSI to open and allow injection.

Main point of all this was to the orignal question about intermittent starting. These systems were known for causing symptoms of good running/no starting with minor drops in fuel pressure.

LenND · Jan 5, 2011

I have a 94 chev 1500 that was cutting out when it got warmed up real good. Chev dealer checked it out --couldn;t find anything wrong. They put in a new dist cap and rotor and it run better but still cut out bad. Got to looking at fuse box under the dash. Replaced two fuses that involved throttle body injection. Runs like a charm hasn't cut out since.

MarkB_MI · Jan 5, 2011

Ya gotta figure out if it's a fuel or ignition problem. Keep a can of starting fluid handy; the next time it won't start give it a snort of ether. If it still doesn't start you know it's an ignition problem.

Bob · Jan 5, 2011

Or simply check it for spark at the plugs when it's throwing a fit and won't start.

If it has spark figure out how to connect a "noid" light to a pair of injector wires at the CPFI electrical connector.

Hobo,NC · Jan 5, 2011

Did he hook a scan tool up and look for a RPM signal,,, it could it be a security issue,,, 54 PSI does throw up a flag,,, he mite scope the fuel pump wave pattern... Sometimes they are just hard to narrow down... Sometimes you wish for a complete failer 8)

Hobo,NC · Jan 5, 2011

The wiring at the connector will just vanish,,, they do'dat

I was thinking when the wiring shorts together it will signal its like 360 degrees and when it goes open it will go to -40 but its late for me... It will for sure dump a load of fuel in it at -40

I will say if it has a big white spot on the side of the coil and i work on it, it will get a new coil... They don't last forever.

Jim In NY · Jan 6, 2011

does it have a fuel pump relay ? If so switch them in the power box and see ?

Chev 1500 intermittent starts

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