1998 GMC Sierra Trouble starting and misses at low RPM

AKAISER30

MEMBER
1998 GMC SIERRA
V8
4WD
AUTOMATIC
200,000 MILES

Truck first started missing out at low rpms and then one day it wouldn't start. First thought it was the crankshaft position sensor but I tried a new one and it did same thing. So I hooked battery charger up to battery and then tried to crank it over to see if it had spark and it about started with one plug wire off. So put wire back on and it started but it missed out when I drove it. It continued to not start ever since unless I have battery charger hooked up. When I try to start it up with out charger it just cranks and its rough sounding and sometimes it hits and about starts but just back fires and never runs. When battery charger is hooked up it usually starts but cranks a few times and acts like timing is off. It used to always start right away. Please give me some advice on what it could be. Thinking something with ignition or distributor or some sensor.

3 Replies

PEAR69

MECHANIC
1,482 POSTS

What is the voltage of the battery? Test the voltage at the actual battery posts, do not just look at the gauge in the dash board. If you do not have a tester take it to autozone or a place like that and they will test it for free.
Test it with the engine off and the engine running.
When you change the crank shaft sensor, sometimes you must get--what they call--a crank shaft sensor re-learn proceedure done at a dealer. I can explain if you want.

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Monday, March 3rd, 2008 AT 5:25 AM

AKAISER30

MEMBER
2 POSTS

Yeah but it wouldnt be the crankshaft sensor would it if it fires once in awhile. Usually a crankshaft sensor is good or bad not in between. And do u think it could be something with distributor or coil or another sensor because my battery is fully charged, but it just helps turn motor over quicker with the battery charger on it so then it sparks better and starts it up. Once running it just misses out. So I dont think it is the sensor. Has to be something else. Any ideas?

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Monday, March 3rd, 2008 AT 12:30 PM

PEAR69

MECHANIC
1,482 POSTS

When your engine was started at the factory for the first time, the computer went through several learning proceedures that are unique to your truck. One of which is the distance the crank sensor is from the reluctor ring on the crank. This distance is learned to.001 inch. As your engine ages, the bearings wear and this distance changes. Normaly the computer can adjust for this, but when you change the sensor sometimes--not all the time--the sensor is a little too far off for the computer to compensate. About 3 out of 10 need a crank relearn done. Crank sensors almost never wear out--what wears out is the main bearings on the crank. Failures of the sensor can be attributed to excessive carbon build up--that's it.� Your problem, however, probably is not the sensor. If you can put the old one back in--if not have the relearn proceedure done and check the following in order--1--battery connections voltage drop--no more that.02 volts.2--spark plug gap and condition--replace with A/C Delco only!3--plug wire resistance--30,000 ohms max--replace with A/C delco only!4--Distributer cap and rotor--replace with any brand5--coil--can be tested at autozone or any parts store for free6--ignition module--test at same place for free7--compression test--no less than 10% difference of highest reading and no less that 110 psi at any cylinder.
Also, the distributers' gear usually wears out around 180000 miles. If all the above check out--take the distributer out and check the gear--I'll bet it is worn out if it has never been replaced. Common failure.

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Monday, March 3rd, 2008 AT 7:56 PM