bartmanC5

iTrader: (3)

My winter vehicle is a 210k mile 97 GMC Sierra with the 5.7L Vortec L31, and recently it cranks slow when hot. Stock but for the MPFI conversion and several intake gaskets. If left overnight or more than 2 hours, it will crank normally (maybe 250 rpm) for maybe 1.5-2 crankshaft revolutions and fires up in less than 1 second. When hot, the first two compression strokes take around 2 seconds by themselves (ie. < 60 rpm), before speeding up and starting just fine.

Did a clamp-on ammeter, and normal cranking is within spec around 150 amps. Slow crank when hot is pulling over 300 amps. I put a new starter in, no change and same amp readings with both starters. Disconnected the coil in case it was an ignition timing issue, same result. Briefly loosened the belt, and Alt, A/C, and water pump are all freely moving.

I'm wondering if I'm (a) headed for a failure, (b) if there's anything I overlooked, or (c) normal for a 200k mile block. Is this worn bearings, lifter issues, or something else? I'm hoping for one more winter (which just started) before upgrading to an newer Sierra or TBSS.

Thanks for any input.

64nova_wagon

iTrader: (1)

Thta sucks.. Starter is what I thought. I know wore bearings will do that when bad.. I would double check all your cables for corrosion.. But sounds internal..

bartmanC5

iTrader: (3)

Thx for the reply. I cleaned up both at the battery with a wire wheel, and the positive cable at the starter and have very low resistance. I also used jumper cables from neg-to-block to rule out grounding issues.

I suppose I can do an oil analysis next time around to see whats going on. No obvious bits last time in the drain pan that I can recall. Are there other symptoms of warn bearings?

Any other suggestions to baby this for 6 more months? I plan on using the block heater and keeping the RPMs down. I'd hate to blow something up.

Big Al

iTrader: (1)

Measure the battery voltage when cranking hot.

Al 95 Z28

bartmanC5

iTrader: (3)

It had cooled down sufficiently since my last drive, so I checked. 12.0V with the DRL lights on, 10.4V cranking at normal speed. I went for a drive to reheat and the hard-crank is 12.1V dipping to 8.8V. I had the ammeter peg past 400A.

I should also add that I've tried cranking hot while jumped to a running 13.5V car with the same result. It cranks slightly stronger than unassisted, but still much much slower than when cold.

Big Al

iTrader: (1)

Here is what I think from your results. The battery is weak. The jumper cables you're using are too small. The cranking current goes up because the voltage is low. Try replacing the battery with a known good one. There is a chance the starter is bad, but I'd almost bet it's not.

If replacing the battery doesn't help let me know. I'll guide you to some other easy tests. BTW, what are you reading the battery voltage with?

Al 95 Z28

bartmanC5

iTrader: (3)

Thanks for the help, Al. It does seem like a weak battery. I once was misled by poor jumper cables many years ago, so the set I've got is much better. That being said, I have often had poor contact with the zig-zag teeth not passing adequate current because of the limited surface area in contact. I've jumped numerous V6 and V8's with them, so I do have faith they work.

The C5 battery is about to be pulled for the winter, so I will have a known good to test with.

Voltage is being read at the terminal posts with an entry level Sears digital VM with a helper turning the key. Amperage is with a Sears clamp-on ammeter. They both report values a couple times a second.

Would a battery load-test shed any light on this? Or is that in essence what I've been measuring? The first 2 or 3 compression strokes draw that massive current, then it spins up to normal speed and draws the normal 150 amps. A carbon-pile load tester set simply to 160 amps would probably pass at 10.4V right? The 10.4V normal cranking test result would be a "pass" based on the stuff I'm reading online.

Big Al

iTrader: (1)

Better to read battery voltage during cranking with a regular meter, not a digital. Lots of "hash" when the starter is grinding. Load test if done with enough load will show the problem. Just watch the dome lamp when you crank it and you'll see what's happening to the voltage. Keep it simple

You may want to do some voltage drop tests but I'd check it with a known good battery before I did anything else.

About jumper cables. I have a go cart that has a 12 volt battery. Had some Harbor Freight jumper cables in my truck and the cart needed a jump. So I hooked it up. It wouldn't crank the 150 cc engine, so I tried the horn. The cables would not even power the frigging horn! So I jumped it with a set of cheap AutoZone cables and it fired right up. So I pulled the insulators up on the HF cables, the wire was about 12 gauge and the crimps were pathetic.

Al 95 Z28

bartmanC5

iTrader: (3)

Thanks again for the reply Al.

I picked up a HF 500A load tester with an analog gauge, and a specific gravity gauge. My battery is from Wally World 690CCA, mfr stamp Sept 2006. Didn't realize it was that old. I compared against my wife's 2010 VW with a "fresh" battery and ran these tests.

Base voltage (GMC) 12.0V
Base voltage (VW) 12.6V
Test #1 (GMC) 150A 15 seconds, lowest voltage 10.5V.
Test #2 (VW) 150A 15 seconds, lowest voltage 10.5V.
Test #3 (GMC) 350A 10 seconds, lowest voltage 8.0V.
Test #4 (VW) 450A 10 seconds, lowest voltage 9.5V.


The dial wasn't changed between tests 3 and 4, so it seems to be telling. Specific gravity for 3 cells was 1.275, 1 @ 1.2, 1 @ 1.21, 1 @ 1.225. Looks like I have a half-alive, half-dying battery on the decline.

I'll pull one of the good batteries just to confirm, perhaps tomorrow.

Big Al

iTrader: (1)

Didn't know HF had anything to test specific gravity. Is it a hydrometer? Their web site search engine sucks big time. The best test is a Refractometer. Sort of a moot point as few batteries have a way to get to the cells. I guess the tops come off your GMC battery? If they do, remove all the tops and have someone crank it. Look for cells with lots of bubbling. Dead sure way to find bad cells.

Al 95 Z28

bartmanC5

iTrader: (3)

My bad, the $5 hydrometer was from Autozone. The battery caps are removable and water level was fine. Tomorrow I'll try a load test with the caps off to look for bubbles. Some bubbling is normal, and I'm looking for excessive bubbles?

In my driving today with 4 or 5 "hot" restarts, none of them cranked slowly. Now with the cooler weather the cranking issues may drop off, but the catalytic efficiency faults have returned. Six more months then out to pasture...