95 C1500 5.7L Stalling at idle, burning oil+coolant, low compression in 2 cylinders

[juniper_berries]

Hello!

I'm trying to diagnose a few issues with my 95 C1500 with the 5.7L TBI engine (4spd automatic). It just cracked 300 000 km. For some background, the truck was sitting for several years and I got it for free. It actually started up okay after sitting for a couple years, but didn't make it far and I wound up replacing the entire ignition system (finding that the issue was that the pickup coil on the shaft connecting to the distributor was shot. Unfortunately I don't have a particularly detailed history for the vehicle.

Since I got it in relatively functional condition, it's always had an issue with stalling at idle. This almost always occurs only while in gear, and it will run rough (RPM is not stable, it surges a bit and sometimes drops down to as low as ~400). Trying to turn the wheel at low/no speed will almost always cause it to stall. It usually runs fine while cold, and these issues typically pop up when the engine gets warm (although not exclusively). There are a few other issues as well that may or may not be related:

1) It burns approximately 1L of oil per month. We don't commute with it, just use it for doing grocery runs/etc.
2) I've recently noticed (last month or so) that it has started burning coolant as well.
3) The stalling has been slowly getting more frequent.
4) It makes a knocking-type noise when its cold, particularly during winter. I've read that this is probably a phenomenon called "piston slap" which is common to this type of engine. Goes away when its hot.
5) I'm not 100% confident that I set the distributor position correctly when I was working on the ignition system. I can connect my computer to the OBD1 port with a program that lets me pull engine sensor data, but I couldn't figure out how to get the timing right with that. So I just rotated the distributor cap with the engine running until it sounded like it was running the best. If there is an easy way to determine whether the timing is screwed up with a computer, that might help.
6) The point that prompted me to make this post - I just ran a compression test and found that cylinders 3 and 5 (the driver's side middle cylinders) have low compression, with cylinder 3 being around 130 PSI and cylinder 5 being around 90. The rest were consistently around 150-160 PSI.

Otherwise, I have recently replaced the following with new, reasonable quality parts:
1) Thermostat and water pump (did a coolant flush as well)
2) Fuel pump
3) AC Compressor
4) EGR Valve
5) Ignition system including distributor (cap and shaft), ignition coil, ignition control module, spark plugs, spark plug wires.

My intuition says that the head gasket is on its way out on the driver's side, but I wanted to see if you lovely folks had any insight before I go pulling bits apart. Any advice or guidance would be much appreciated!

 

[Schurkey]

95 C1500 with the 5.7L TBI engine (4spd automatic).

I wound up replacing the entire ignition system (finding that the issue was that the pickup coil on the shaft connecting to the distributor was shot.

Ignition system including distributor (cap and shaft), ignition coil, ignition control module, spark plugs, spark plug wires.

Did you replace the ENTIRE distributor, or just the cap, mainshaft, and module?

it's always had an issue with stalling at idle. This almost always occurs only while in gear, and it will run rough (RPM is not stable, it surges a bit and sometimes drops down to as low as ~400).

Connect a scan tool, verify the Idle Air Control system.

Trying to turn the wheel at low/no speed will almost always cause it to stall. It usually runs fine while cold, and these issues typically pop up when the engine gets warm (although not exclusively).

Again, check the IAC. But, when cold, the richer fuel mixture and supposedly higher idle speed would cover-up vacuum leaks and poor compression.

1) It burns approximately 1L of oil per month. We don't commute with it, just use it for doing grocery runs/etc.

How many miles per month? I get the sense that it's "not many", which makes a liter of oil quite excessive.

Are you sure it's being lost to burning, and not leakage?

That's gonna be hard on the catalyst. You can kinda expect to have to replace the cat(s). Are the spark plugs getting fouled with oil?

2) I've recently noticed (last month or so) that it has started burning coolant as well.

Again...lost to the combustion chamber, not to leakage?
Are the plugs getting "cleaned" from coolant? The quick-connect at the heater hose, right rear of the intake manifold is a common leak-point.

So are faulty intake gaskets. TBI intake gaskets are "special" and must be used if the TBI intake manifold is used.

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4) It makes a knocking-type noise when its cold, particularly during winter. I've read that this is probably a phenomenon called "piston slap" which is common to this type of engine. Goes away when its hot.

"Goes away when hot" is a characteristic of piston slap. The SBC is not the worst piston-slapper, but I suppose anything is possible.

5) I'm not 100% confident that I set the distributor position correctly when I was working on the ignition system. I can connect my computer to the OBD1 port with a program that lets me pull engine sensor data, but I couldn't figure out how to get the timing right with that.

There's no way to know what the ignition timing is set to, using a scan tool. You need a timing light, or magnetic timing indicator. The magnetic indicator is VERY uncommon, but wonderful to work with if you had access to one. The OEM timing cover should be set-up for a magnetic probe, and the offset will be 9.5 degrees.

Snap-On "Lumy/Mag II" Diesel/spark ignition timing probe system. Magnetic wand, inductive pickup for #1 plug wire, or--not shown--luminescent probe for the glow-plug hole for Diesel compatibility.

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So I just rotated the distributor cap with the engine running until it sounded like it was running the best. If there is an easy way to determine whether the timing is screwed up with a computer, that might help.

Can't be done with the computer/scan tool. As said--timing light or Lumy/Mag.

You NEED to do this before getting wound-up on head gaskets and such. However, don't expect proper ignition timing to fix oil consumption or coolant loss.

6) The point that prompted me to make this post - I just ran a compression test and found that cylinders 3 and 5 (the driver's side middle cylinders) have low compression, with cylinder 3 being around 130 PSI and cylinder 5 being around 90. The rest were consistently around 150-160 PSI.

150--160 sounds real normal to me. Altitude makes a difference, as does cranking speed. Therefore, battery charge and battery capacity makes a difference.

130 doesn't worry me too much. 90 does. But if the head gasket were popped between those two cylinders, they'd both read about the same, and much lower. You'd also have backfiring/knocking as #3 cylinder ignited the mixture in #5.

With just #5 being horribly low, I'd be looking at a cylinder leakdown test to see if this is rings or valves/valve seats.

My 5.7L "Caprice" crate engine popped the head gasket between #3 and #5. Gouged the block, gouged the head even worse. No coolant loss, and no history of oil consumption.

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[juniper_berries]

Did you replace the ENTIRE distributor, or just the cap, mainshaft, and module?

As far as my understanding of distributors goes, I bought the whole distributor. This image, while not the exact part I put in, is very similar. I did NOT use the included aftermarket ICM and replaced it with an OEM one instead.

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Connect a scan tool, verify the Idle Air Control system.

I'm travelling to my parents' house tomorrow and I'll connect my computer and try tunerpro. I otherwise don't have access to any fancy scanning systems like the SnapOn brick.

How many miles per month? I get the sense that it's "not many", which makes a liter of oil quite excessive.

I'd say roughly 250-300 miles per month, excluding trips to go visit family (but I top up the oil before every trip anyways). Hard to pin down exactly, but yeah the oil consumption felt quite excessive.

Are you sure it's being lost to burning, and not leakage?

I have two suspect spots for leaks. First is on the driver's side cylinder bank, second is around the front of the engine. Looking at it now, the front could be a slow coolant leak from the radiator house right above it... But intuitively I would expect to see more oil if it was leaking a litre per month.

Photos:
Driver's side cylinder bank

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Front of engine:

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That's gonna be hard on the catalyst. You can kinda expect to have to replace the cat(s). Are the spark plugs getting fouled with oil?

No carbon/oil fouling of the plugs. They seemed fine.

Again...lost to the combustion chamber, not to leakage?
Are the plugs getting "cleaned" from coolant? The quick-connect at the heater hose, right rear of the intake manifold is a common leak-point.

Checked the quick connect. Looks okay. Plugs didn't look particularly "cleaned", although I don't know exactly how that would manifest. They were stained light brown mostly.

So are faulty intake gaskets. TBI intake gaskets are "special" and must be used if the TBI intake manifold is used.

There is no evidence of coolant leaking and pooling anywhere between the cylinder banks/on top of the intake. If the gasket is leaking and coolant is pooling under the intake I'd imagine it may not be externally visible and the intake would have to be removed to check. Is this the case?

There's no way to know what the ignition timing is set to, using a scan tool. You need a timing light, or magnetic timing indicator. The magnetic indicator is VERY uncommon, but wonderful to work with if you had access to one. The OEM timing cover should be set-up for a magnetic probe, and the offset will be 9.5 degrees.

Ah okay, this seems more sensible to me. I think I read some overcomplicated or incorrect information elsewhere, something about reading the advance/retard angle from the ignition control system through the scanner.

130 doesn't worry me too much. 90 does. But if the head gasket were popped between those two cylinders, they'd both read about the same, and much lower. You'd also have backfiring/knocking as #3 cylinder ignited the mixture in #5.

With just #5 being horribly low, I'd be looking at a cylinder leakdown test to see if this is rings or valves/valve seats.

Okay, yeah I'll run a leakdown test as well. I don't have a kit for it, but its simple enough that I may be able to rig something up.

Thanks a ton for your helpful suggestions and comments. Very much appreciated.

 

[Schurkey]

As far as my understanding of distributors goes, I bought the whole distributor. This image, while not the exact part I put in, is very similar. I did NOT use the included aftermarket ICM and replaced it with an OEM one instead.

Yup. Entire distributor, and you wisely didn't use the included module.

I'm travelling to my parents' house tomorrow and I'll connect my computer and try tunerpro.

OK.

I'd say roughly 250-300 miles per month, excluding trips to go visit family (but I top up the oil before every trip anyways). Hard to pin down exactly, but yeah the oil consumption felt quite excessive.

A liter in 300 miles is wildly excessive.

I have two suspect spots for leaks. First is on the driver's side cylinder bank, second is around the front of the engine... ...But intuitively I would expect to see more oil if it was leaking a litre per month.

Yes, you have some seepage. But that's not evidence of a liter in 300 miles.

No carbon/oil fouling of the plugs. They seemed fine.

That's kinda unusual for oil consumption that bad. How long were the plugs in? How many miles/years?

Checked the quick connect. Looks okay. Plugs didn't look particularly "cleaned"

The next thing to check is to see if the oil gets "Chocolate milkshake" colored--evidence of coolant in the oil.

Or pull the drain plug after the engine has sat overnight, and the oil would be floating on top of the coolant. If you pull the plug, and coolant runs out instead of oil, you know the coolant leak is internal--into the oiling system.

There is no evidence of coolant leaking and pooling anywhere between the cylinder banks/on top of the intake.

So no evidence of external coolant leakage. No spots on the ground, no sickly-sweet smell in the cab--along with foggy windows?

I read some overcomplicated or incorrect information elsewhere, something about reading the advance/retard angle from the ignition control system through the scanner.

Yup. That started in '96 with the OBD2/Vortec system. Does not apply to your '95/TBI. And it's not checking ignition timing, it's checking the synchronization of the cam sensor signal against the crankshaft sensor signal--and your engine doesn't have a crank sensor.

Okay, yeah I'll run a leakdown test as well. I don't have a kit for it, but its simple enough that I may be able to rig something up.

Possible, and potentially helpful. Not as helpful as a properly-instrumented leakdown tester.