Slow Crank Seems to have Mechanic Puzzled Too

I may have posted a year ago or so about my odd slow crank issue on my K1500, but here's an interesting read/update that seems to have the mechanic puzzled too. It has the 5.7 Vortec and a direct drive starter. When I drive it around town or on the interstate and stop for a short while (5-10 minutes) and then hop back in and go to start it, it cranks over slow...so low that after the first crank it seems to hang. The second crank or so it usually starts, but it acts like I might be stranded because of how long the hang lasts.

I replaced the starter (with another direct drive) in July along with the positive battery cable, did a tune up in in in August (plugs, wires, cap, rotor) and had the battery replaced a year ago. All this needed to be done anyway for various reasons, so not a big deal.

This past Monday I took it to a well known, trusted mechanic in town. They've been doing various things to it to track the problem down. They replaced the battery since the battery went bad (this is the third one in a year and a half! How??), and the negative battery cables. It's still doing the slow crank. They also said it doesn't exhibit the symptoms of a faulty crankshaft position sensor.

Next thing they will be testing tonight/tomorrow is something in the distributor. Has anyone else heard of this or experienced it? It seems really odd...my dad's '88 TBI has never had a slow crank issue with a good battery, but this Vortec seems to really enjoy being bizarre about it.

3 batteries in a year and a half signals to me, charging system problems, it is possible that the alternator is overcharging the batteries, and boiling them.

All cable connections need to be clean and tight in order to get maximum voltage to the starter. First, I would check the float voltage in the battery after a charge, and if it is a maintenance type battery, pop the tops with a screwdriver and verify the electrolyte is over the top of the plates in each cell. If you don't have a battery hygrometer, perhaps an auto parts store can test each cell in the battery, to see if there is issues with a dead cell, or you can purchase a hygrometer, they are inexpensive.

On the other hand, it may be a heat related issue, perhaps the catalytic converter is becoming plugged or melted, the exhaust temperature elevated, and the starter is cranking slow from the heat radiating into it.

There could be many factors, but I would start with a full charging system diagnosis, but check the battery first.
Even if it is new, that doesn't mean rule it out. A bad battery can cause an alternator issue, and the reverse is also true.

Thanks for the input! I should clarify that the last battery that died last October was a result of the alternator testing bad, so both the alternator and batter were replaced. The mechanic has been focusing on the charging system so far, and it sounds like they'll probably rule that out after a couple more tests.

I never thought of the cat being plugged. What's a good way to test the cat, with an IR heat gun on the cat, manifolds, or both?

michael hurd said:

3 batteries in a year and a half signals to me, charging system problems, it is possible that the alternator is overcharging the batteries, and boiling them.

All cable connections need to be clean and tight in order to get maximum voltage to the starter. First, I would check the float voltage in the battery after a charge, and if it is a maintenance type battery, pop the tops with a screwdriver and verify the electrolyte is over the top of the plates in each cell. If you don't have a battery hygrometer, perhaps an auto parts store can test each cell in the battery, to see if there is issues with a dead cell, or you can purchase a hygrometer, they are inexpensive.

On the other hand, it may be a heat related issue, perhaps the catalytic converter is becoming plugged or melted, the exhaust temperature elevated, and the starter is cranking slow from the heat radiating into it.

There could be many factors, but I would start with a full charging system diagnosis, but check the battery first.
Even if it is new, that doesn't mean rule it out. A bad battery can cause an alternator issue, and the reverse is also true.

Click to expand...

Does it crank fine when the truck is cold? If its only after getting warm, it could be the old heat soaked GM starter issue.

Rogue said:

Thanks for the input! I should clarify that the last battery that died last October was a result of the alternator testing bad, so both the alternator and batter were replaced. The mechanic has been focusing on the charging system so far, and it sounds like they'll probably rule that out after a couple more tests.

I never thought of the cat being plugged. What's a good way to test the cat, with an IR heat gun on the cat, manifolds, or both?

Click to expand...

It depends on the range of your IR heat gun... it may register hi if over the limit for display range.

In general if the converter is working properly you will have about 100 F hotter at the outlet compared to the inlet.

Does the exhaust have a noticeable rattle from a broken catalyst brick?

Even if you have no rattle it may be partially melted, and causing a restriction in flow. Depending on the muffler design, you might have a baffle partially collapsed even.

I have never used a FLIR camera but that may also be an option to see what the exhaust looks like after it is hot from driving.

On a friend's truck identical to mine, down to engine, transmission option, gearing and tires his catalyst was damaged.

The symptoms were not very noticeable around town, but accelerate up to highway speed and it was a dog compared to mine.

He swapped out his stereo head unit as it was cutting out, thought the pre amps were acting up. Turns out the floor became hot enough to blister the paint on the inside of the cab, and the RCA coax cables to the rear mounted amps were shorting. The jacket on the cables had melted and was stuck to the back of the carpet.

This was found after a bit more sleuthing, when the replacement cd player acted in the same manner as the old one.

One thought that comes to mind, does your vehicle 'use' any coolant? It is possible that the engine is leaking coolant into a cylinder when hot.

Sounds like a voltage drop caused by excessive resistance in the starting circuit. A few minutes with an amp clamp and a multimeter should have had this solved, or very close to it.

michael hurd said:

One thought that comes to mind, does your vehicle 'use' any coolant? It is possible that the engine is leaking coolant into a cylinder when hot.

Click to expand...

No, the truck hasn't used coolant since I had the intake manifold gaskets replaced two years ago. Ever since I replaced my cracked radiator, it hasn't ever been low on coolant.

SAATR said:

Sounds like a voltage drop caused by excessive resistance in the starting circuit. A few minutes with an amp clamp and a multimeter should have had this solved, or very close to it.

Click to expand...

I talked to him on the phone today and yesterday, and he seems to know his stuff. I'm familiar with electrical circuits as I have some background in it, and he wasn't BS-ing me. The mechanic was going over the starting circuit with at least a multimeter, and even replaced the grounding cables to try when there was some excessive resistance in them. He's had these before and correcting excessive resistance in the starting circuit has always fixed this symptom. I face palmed myself for not checking the resistance myself on them last weekend when doing some other work on it.



I'm curious to look into this possibility of a plugged/restricted cat, as it hadn't crossed my mind. If I get it back tomorrow I may have to get it up to temp this weekend and pick up an IR gun. On a side note, could excessive leakage from am injector cause this? I think I still have the original SFI kit on the engine (another thing to replace at some point in the future with the newer spider injection style). I know those are prone to failure, but the injector that failed on my mother's '98 Blazer caused misfire codes on the vehicle and I haven't had any of that.

Rogue said:

Thanks for the input! I should clarify that the last battery that died last October was a result of the alternator testing bad, so both the alternator and batter were replaced. The mechanic has been focusing on the charging system so far, and it sounds like they'll probably rule that out after a couple more tests.

I never thought of the cat being plugged. What's a good way to test the cat, with an IR heat gun on the cat, manifolds, or both?

Click to expand...

I would test it with a sawzall and a piece of plain exhaust pipe

90halfton said:

I would test it with a sawzall and a piece of plain exhaust pipe

Click to expand...

The thought has crossed my mind...but I like a quiet exhaust and not getting stopped by some of the police in my town. My friends put time and money to make their exhaust louder, I put time and money into mine to make it quieter