'98 4.3 blazer intermittent starting problem

Hi everyone, I'm going to try and describe the problem I'm having. Up until a month ago, this truck would start with just a bump of the key. Never had a problem with it other than the fuel pump going tits up about 2 years ago. Now sometimes it will crank right up just as it always has and sometimes now I have to try several times for it to catch and run. I have noticed that if it doesn't crank as soon as I turn the key it's not going to crank no matter how long I let it spin over. I can turn the ignition off and try again and it fires right up.

I put a fuel pressure gauge on it last night and it primed to 61 lbs pressure, after it starts it falls to about 50 lbs and holds steady. Also did a static leak down test and the pressure is holding steady. I let it sit for about 2 hours and checked it and it had dropped to about 45 lbs. but there is no immediate drop in pressure, and no fuel leaking down inside the throttle body.

After reading page after page on this forum I have yet come across an issue like the one I'm having. I've tried to find info on the starting sequence but have yet to find a clear explanation of it. So if anyone knows I would love to hear it.

So far I'm suspicious of the crank or cam position sensor and possibly the ignition switch. What do you guys think?

Rod

Several things could cause what you're describing.

First thing to check is fuel pressure and leakdown, it is critical on this engine. I realize you checked it, but use this method for the most accurate readings: With the pressure tester connected to the service port near the distributor, ignition switch in the OFF position, install a fused jumper wire from the battery positive terminal to the fuel pump prime terminal in the underhood fuse panel:
http://i561.photobucket.com/albums/ss58/cwhook/DSC_0154-Copy-1.jpg
Fuel pressure should immediately and consistently indicate 60psi to 66psi, try it several times while watching the tester. When you remove the jumper, pressure must remain above 55psi for at least 10 minutes. If it fails either part of this test, there is a problem with the fuel delivery system. It does NOT identify the problem, further testing is necessary to pinpoint the problem. GM does not publish any "engine running" fuel pressure specs for this engine...don't waste your time, they mean nothing.

Next thing is distributor cap, rotor and spark plugs. These engines are really fussy when it comes to tune up parts.... use AC Delco only. Use a good quality plug wire set... AC Delco is fine, but Autolite Professional Series wires have better specs and are considerably less money. Put a dab of silicone dielectric grease on each terminal, inside and out, of the cap. AC Delco plugs 41-993 with a gap of .060" is recommended.

Post your results.

Hi Old Master, and thanks for the reply. My wife uses this truck for work and it is gone pretty much from dawn till dusk, but I will do what you suggested this weekend and post the findings. I really appreciate your taking time to reply.

Hi Old Master, hope you're having a great day so far. Replaced plugs, wires, rotor, and cap. I really believe that was the original cap and rotor, still had a seal sticker on the backside of the cap. Truck has 210,000 miles and still runs smooth.

I did the fuel pressure procedure above, actually three times. The first time, the engine was cold, used the jumper wire and pressure went to 58 psi, released the pressure, again went to 58 psi, let it set for 10 min., may have lost 2 psi, never did get below 55psi. Cranked the engine, it stumbled just a second but it did catch and run. I did the same test again after driving it to warm the engine, Pressure went to 62psi and only lost about 2 psi over ten min time. have cranked the engine several times since then and it seems to be cranking fine so far. Why the difference in fuel pressure between cold engine and a warm engine? I'll let her drive it for a day and let you know how it goes. Thanks Again

The reading of 58psi is too low, and low fuel pressure will cause hard starting but, it could be due to the battery being low after sitting over night. Or there might be a draw on the battery, or it may just be getting weak. Could also be a poor connection for the fuel pump, or the ground for the pump. While the engine warms up, the alternator is charging the battery causing voltage in the battery to rise, which will cause fuel pressure to increase. If you have a battery charger, connect it and leave it connected until the next cold start attempt, and see what fuel pressure is. There have been some instances when the ignition switch fails, not allowing battery voltage to various circuits in the vehicle. The grey wire at the fuel pump module must show battery voltage while cranking the starter. Both black wires at the module must show less than 5 ohms resistance to ground at all times. Post your results and we'll go from there.

I replaced the battery when this problem first started about a month ago. The battery was 18 months old with a 2 year free replacement so they checked it and said it was defective so they replaced it for free. Alternator is charging fine, showing just a little over 14v while running. The under hood light was on most of the day today, about 5 hours, would that be enough drain to cause a voltage drop?

When you say "the wires going to the fuel pump module", do you mean the fuel pump itself that's in the tank?

Ideal charging voltage is 14.2 volts, so you're good there ;) The underhood light is on a timer and should have shut off on its own after ~10 minutes or so. Yes, that would drain the battery if it didn't shut off. Does it shut off when you lower the hood? Shouldn't need to close it all the way to see it turn off. Checking voltage and resistance at the module connector, unfortunately requires dropping the tank down a few inches to reach the connector.