Rebuilt Engine - Low Oil pressure & Slight Overheating

[Garrick2007]

So i did what i would call a partial rebuild on my M20 engine (230k miles) due to smoking from foreign objects entering the cylinders, in my case spark plug heads. The head was rebuilt 60k ago so I honed cylinders, replaced piston rings, rod bearings (just because, old ones looked good) timing belt, water pump, thermostat. Car runs great, no oil loss, no smoking but when it gets up to operating temperature my oil light comes on at idle and goes away under any load. Oil pressure was never an issue before. Since then i dropped the pan again and plastigauged one of the rod bearings to check for clearance since i missed that step originally and all checked out, i also opened up the oil pump and gave it a good clean, had lots of junk and gasket maker in the filter. Im running 20-50w and I've changed oil twice so far along with filters. Oil pressure sensor was also replaced. With that said the car also overheats a tad, it will past the mid point often, its not to the point where i can't drive it in hot weather, I've driven it in 100 degree weather over an hr but i think if left idling long enough the temps will keep rising. I have no leaks what so ever, my oil is not mixing with my coolant. Ive also re-flushed the coolant system and everything. What should i start looking into now? Ive heard that a improperly bled coolant system can cause low oil pressure at idle. I think this really may have something to do with it because the car runs great aside from this and those are the only 2 issues that are lingering. Since the rebuild I've put at least 1200 miles on the car. Going to do another oil change any day now. Any suggesting id love to hear, thanks a lot!

[jlevie]

Plastigauge isn't a reliable method of checking bearing clearances on on BMW engine. The only reliable method is to measure each rod journal with a tenths reading micrometer and use that measurement to pick the "color" of rod bearings that provide the correct clearance. Depending on wear more than one "color" bearing may be needed. Incorrect rod bearing clearances may cause your low oil pressure warning on a hot engine.

The overheating is a different issue and may well be due to improper bleeding, wrong/bad thermostat, or bad fan clutch.

1) Using ramps or a jack get the front of the car 1' or more higher than the rear. That will make the radiator and bleed screw the highest point on the engine and facilitate removal of air.

2) With the bleed screw open, add coolant until no more air comes out of the bleed.

3) Leave the filler cap off, or at least loose, and set the heat for max temp and fan speed. Leaving the cap loose will prevent air that's still in the system from causing a "coolant fountain" once the engine heats up. Start the engine and allow it to warm up to operating temp. As it warms up occasionally crack the bleed screw to release any air and top up the coolant as necessary.

4) Once the engine is at temp bring it up to 2000-2500rpm for a few seconds several times. Then crack the bleed until no more air is released. At this point the heater should be throwing lots of hot air, which indicates that the heater core is filled with coolant. You may have to repeat this a few times to get all the air out.

5) Drive the car a bit, allow it to cool back down, and recheck the bleed for air. Over the next few days you may get very small amounts (a few bubbles) of air out of the bleed screw.

A vacuum fill bleed system makes filling and bleeding the cooling system a trivial task. But the procedure above will work if you don’t have access to a vacuum fill system. On an E30 and some other BMW’s you can speed up the fill process by removing the small hose that runs from the coolant reservoir to the top of the radiator at the radiator end. Then fill the reservoir to the mark, screw on the cap and blow into the hose until coolant comes out of the radiator nipple. That will fill the engine and radiator with coolant pretty quickly. It may take more than one fill/blow cycle to fill the radiator. Then bleed as per above to get any remaining air out.

If the oil pressure warning only occurs at idle when the engine is overheating, it could be from worn main bearings, worn intermediate shaft bushings, worn cam/head bearings worn oil pump, or a bad oil pressure relief valve. Fixing the overheating problems and/or running a heavier weight oil could be a short term solution.

[TobyB]

Everything Jim said.

I'd take one step back,
and fit a proper oil pressure and temperature gauge.

Then re- evaluate.

I'm less familiar with the M20, but on the M10, if your oil
pressure is low, the relief valve is a common culprit- it can
stick, or junk can bind it up.

t

[digger]

I'd take one step back,
and fit a proper oil pressure and temperature gauge.

Then re- evaluate.

i agree with this, i had a brand new switch start playing up in under 1000miles on a full rebuild. i dont buy the cheap crap brands anymore. it seems unlikely that it is a switch issue though given the symptoms are when it is hot but its a relatively cheap and simple test.......

[e30davie]

Plastigauge isn't a reliable method of checking bearing clearances on on BMW engine.

Out of curiosity, why is this?

I've never actually done plasti gauge, i must admit from what i have seen it seems a bit agricultural. I plan to do a bottom end in the future, but i will probably just get my machine shop to check the crank and supply bearings to suit.

[Garrick2007]

Plastigauge isn't a reliable method of checking bearing clearances on on BMW engine. The only reliable method is to measure each rod journal with a tenths reading micrometer and use that measurement to pick the "color" of rod bearings that provide the correct clearance. Depending on wear more than one "color" bearing may be needed. Incorrect rod bearing clearances may cause your low oil pressure warning on a hot engine.

The overheating is a different issue and may well be due to improper bleeding, wrong/bad thermostat, or bad fan clutch.

1) Using ramps or a jack get the front of the car 1' or more higher than the rear. That will make the radiator and bleed screw the highest point on the engine and facilitate removal of air.

2) With the bleed screw open, add coolant until no more air comes out of the bleed.

3) Leave the filler cap off, or at least loose, and set the heat for max temp and fan speed. Leaving the cap loose will prevent air that's still in the system from causing a "coolant fountain" once the engine heats up. Start the engine and allow it to warm up to operating temp. As it warms up occasionally crack the bleed screw to release any air and top up the coolant as necessary.

4) Once the engine is at temp bring it up to 2000-2500rpm for a few seconds several times. Then crack the bleed until no more air is released. At this point the heater should be throwing lots of hot air, which indicates that the heater core is filled with coolant. You may have to repeat this a few times to get all the air out.

5) Drive the car a bit, allow it to cool back down, and recheck the bleed for air. Over the next few days you may get very small amounts (a few bubbles) of air out of the bleed screw.

A vacuum fill bleed system makes filling and bleeding the cooling system a trivial task. But the procedure above will work if you don’t have access to a vacuum fill system. On an E30 and some other BMW’s you can speed up the fill process by removing the small hose that runs from the coolant reservoir to the top of the radiator at the radiator end. Then fill the reservoir to the mark, screw on the cap and blow into the hose until coolant comes out of the radiator nipple. That will fill the engine and radiator with coolant pretty quickly. It may take more than one fill/blow cycle to fill the radiator. Then bleed as per above to get any remaining air out.

If the oil pressure warning only occurs at idle when the engine is overheating, it could be from worn main bearings, worn intermediate shaft bushings, worn cam/head bearings worn oil pump, or a bad oil pressure relief valve. Fixing the overheating problems and/or running a heavier weight oil could be a short term solution.

Thanks a lot! I'm probably going to try the vacuum full bleed soon because I've tried the method above twice from scratch so far, I've also tried filling from the hoses as well. I'm starting to think maybe it's the thermostat, I doubt I was sent a bad pump but who knows honestly. I might put the old one back in and see what it does if the vacuum bleed fails.

As far as worn bearings and pump, my oil system worked perfectly before the rebuild. I never had any issues with oil pressure, my light would only come on breifly at start up or at a near stall out situation so realistically that more then likely wouldn't be the issue right? And I'm already running 20-50 is there ticker I can try?

[Garrick2007]

i agree with this, i had a brand new switch start playing up in under 1000miles on a full rebuild. i dont buy the cheap crap brands anymore. it seems unlikely that it is a switch issue though given the symptoms are when it is hot but its a relatively cheap and simple test.......

Switch is new, replaced it after having these problems.

[Garrick2007]

Everything Jim said.

I'd take one step back,
and fit a proper oil pressure and temperature gauge.

Then re- evaluate.

I'm less familiar with the M20, but on the M10, if your oil
pressure is low, the relief valve is a common culprit- it can
stick, or junk can bind it up.

t

Anyway to check if it's faulty? I remember pushing the stem and it didn't stick or anything. I'd hate to drop the pan again, and now I finally have it to where it's not leaking but if I do I'll probably replace that and the pump.

[ForcedFirebird]

20-50 oil is vague. If the car is running hot enough for the oil temps to approach the 300f mark, conventional oil thins out to water consistency and will trip a stock light (or even a flicker). With a mechanical gauge on a hot FL summer day, we would see ~5-6psi idle after a 40min track session using Diesel truck conventional oil (Delvac), ~10psi with Mobil 1 15-50 synthetic, then finally we switched to LiquiMoly Race Tech 10-60 and the pressure never drops below 16 at idle, even after several hours of summer endurance racing. The pressures mentioned were in the low 300f area, don't remember seeing over 320.

You may be running synthetic, so another thing top check is the oil pressure relief. The hexagon thing sticking down comes off easily with a twist, then open it and inspect the spring/plunger. If you had a lot of sealant in the sump screen, wouldn't doubt the plunger might too. Some local guys block the relief, and haven't seen any real negative effects from this. One of them made 165whp in his +.5mm spec engine that has 4 years on it. But it is strange to see cold pressures in the 70's. When dyno-ing his car, was unaware of the block off, thought something was wrong at first. heh.

I have never experienced the "m20 bleeding issues" often talked about. I use a large mouth funnel that fits the expansion opening tightly, then fill above the cap height. This will make sure the fill level is the highest point in the system, so it will naturally bleed - even if the bleed screw isn't loosened. In a pinch at the track on uneven surfaces, I will pull the tank out of the holder and lift it up while filling. Same effect.

Another thing I like to do when replacing t-stats is drill a 3/16" hole inside the o-ring seal, and put that at the top when installing. Old trick from the GM FWD days, they were really a bear to bleed, carried that over to the m20.

One time, we had heating issues, everything checked out, did a whole day of practice Friday, over heated in two laps Sat with ambient temps ~20f. Found the small hose from rad to expansion had a tiny piece of debris in it, and was blocking enough to cause problems when the stat was closed. Might be completely unrelated since yours is heating all the time, and temps should have had stat open.

Since you recently did a light rebuild, did you happen to check the condition of the intermediate shaft bearings? If they are failing, that's a huge hole in the system to lose pressure.

Really curious to the final diagnosis, please keep us updated.