Resolved Poor Cooling at Low RPM

[Cameron Morison]

I came across an issue on the dyno yesterday, immediately after a run the temps would start to climb from the normal low 90's (celsius) up to around 110 if left idling, however if we held the revs up to say 2000 for a minute then the temps hardly rose.

So my theory is here that there is not enough flow around the cylinder block or cylinder head for managing heat soaking through after a run at idle speeds. I have never had this issue on the race track - always have had good temps but I've recently just replaced a head gasket (if that has any chance of changing things at all)

Possible solutions Ive thought of so far are:
- electric water pump
- electric booster water pump (on demand, run only at low RPM when coolant temp is high)
- change pulley ratio for water pump (Increased chance of cavitation at high RPM)
- change water pump to better design impeller (Im pretty sure its got a open vane impeller currently, it was aftermarket replacement 4 years ago)
- Increase Rad cap pressure (currently seems to vent at 10psi based on the logs)

My system isn't quite stock:
- Twin pass 300x600x76mm cross flow radiator (more drag than an OEM one)
- Single drive belt for alternator and water pump, drives off the outer crank pulley and onto what I think is the larger of the two pulleys attaching to the WP.
- Has a pressurised expansion tank rather than a recovery bottle.

Any suggestions or DSM wisdom that can be offered?

 

[Jonnyeclipse]

Do you know what brand pump you are running and if it has a cast impeller or stamped steel. The cast ones suck for low RPM flow.

 

[Cameron Morison]

I'm pretty sure it was a GMB, and if my memory is any good it's a pressed steel one but I might have to confirm

 

[wparks]

Funny enough, I have had this exact same problem on my Subaru Forester- Running up miles of dirt county roads at 25mph the eco-friendly transmission always upshifts to the tallest gear it can get away with, and the engine runs at a low RPM and I can watch the temperature gauge slowly rise hotter and hotter than it does in normal driving. I pull over, rev the engine up to about 2000 RPM or so for just a moment, and the temperature falls like a rock. Has to be the momentary increase in coolant pressure. I have not fixed it yet, but my first suspicion is that the thermostat is flakey and not opening properly, or the little bleed assembly on the side of the thermostat is not letting enough coolant by to let the thermal valve react when the engine temperature increases. I suggest a good cooling system back flush, and a new thermostat. The cap pressure also seems a little low- I'm used to seeing 14 or 17 pounds pressure- the more pressure, the better a coolant system cools. On a side note- On really old industrial Chevy engines the guys used to pull the little wiggle pin stopper completely out of the bleed hole on the flange of the thermostat so with the more bypassing flow the thermal valve could sense the higher temperatures under short term heavy load and sooner and open faster. I did this on my little Isuzu truck and it was overkill- I froze my butt off all winter as it seemed to open the thermostat too often and the engine coolant never got warm enough for the heater to be comfortable. I suspect there is some balance tradeoff that could be made there if you wanted to tinker with your thermostat for your specific use model- faster sensing versus ability to hold higher engine temperatures in winter.

 

[Cameron Morison]

Working on this further

I think there is something physically wrong. Even just idling it up to temp, it carried on and got hot, I shut it off at 100 and could hear water bubbling.

I had the expansion cap off to look at the level and once approaching 100 it quickly rose before a large air bubble came put. I suspect boiling occurring inside the engine, probably the block.

It never used to have any cooling issues. I suspect either water pump is failed, lost fins or slipping impeller, or radiator is blocked, however it's a race car and gets coolant changed frequently so I think that one is unlikely, along with the radiator only being a year old.

Thermostat begins opening at 82 and is wide open by 95, so seems to be operating normally

 

[Vegas Smith]

My guess would be water pump since it's unlikely to be thermostat or radiator. Only so many things it could be

 

[pauleyman]

If you're boiling at 100 what are you using for coolant?

 

[#95GS-EX]

I have the same issue. Please update with solution. I have changed my thermostat, radiator cap, radiator hoses, I now have mishimoto radiator, reservoir and gates water pump. also burped the coolant system and put oem mitsubishi coolant. and still runs hot until I rev engine past 2000rpm I also have heat blanket on turbo and manifold and vented carbon fiber hood so I know my engine bay has plenty of air circulation

 

[Cameron Morison]

Update, is fixed.

I checked the water pump, was mint. It's a pressed steel GMB one. I tightened the clearance between the impeller and housing by tapping it down the shaft. Should help prevent cavitation too.

Radiator was sparking clean inside and no flow restrictions

I replaced the thermostat with a genuine mitsi one and modified the top hose fitting so I can fill it through that, which means there shouldn't be any chance of air locks

I use water with redline water wetter for coolant. It doesnt increase the boiling temp like glycol, but has the best coefficient of heat transfer. Glycol also makes race tracks slippery if you leak.

I think the cause was either an air lock causing the thermostat to not have hot water on it and subsequently not open, or the thermostat may have jammed. The air lock was potentially introduced by a head sealing issue or head lifting event.

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[1990TSIAWDTALON]

Thanks for the follow up!

 

[chrisman287]

Do you burp the system after replacing coolant?

Edit: Sorry just realized you don’t have a rad cap

 

[#95GS-EX]

I believe my issue was fixed. My shop installed push fans instead of pull fans. im currently running

Hayden High Performance 14 Inch 1761 cfm Electric Puller Fan​

and they are awesome!

 

[AWD-Tony]

I believe my issue was fixed. My shop installed push fans instead of pull fans. im currently running

Hayden High Performance 14 Inch 1761 cfm Electric Puller Fan​

and they are awesome!

Are you running one? Glad it was a simple fix.

 

[Mech Addict]

I’m surprised you run pure water. Transfer coefficient may be a tad better, but you miss out on the boiling point elevation of a 50-50 mix. 223F @ 1atm, 265F @ 15psig. I presume you don’t have freezing concerns given your uses.

 

[#95GS-EX]

Are you running one? Glad it was a simple fix.

running 2 now car temp stays down even with a/c at full blast

 

[Cameron Morison]

Bit more info for the thread.

It ended up not being resolved at all. I triple checked bleeding of the system, went for a drive and within a couple minutes of starting the drive I was up to 95 deg C, but the issue really showed face after about 5 minutes driving when I just held it against the brakes in 3rd gear, just on a measly 16psi boost for maybe 5 seconds. Coolant temps went through the roof, pretty quickly went up to around 115 degrees and they wouldn't recover afterwards so I pulled over and got towed home by a mate.

Anyway, to cut a long story short, I took head off again yesterday:

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[Mech Addict]

crack in the cylinder? That's not good.
J-B weld time.

 

[Kryndon]

Jesus lord almighty that there is a nasty crack! Kudos to you for exhausting all possibilities of your issue, but really unfortunate it turned out to be a cracked block. I don't know if that can be fixed at all, only thing that comes to mind is sleeving the block but at this point you would be better of finding a standard 6 or 7 bolt block off any other Mitsu of that era.

 

[Cameron Morison]

Ive got a stock bore N/A 6 bolt there now, which is going to get used. I wish I could keep it stock bore, but to do that feasibly Id have to buy a set of pistons and accept the risk that they may not fit depending on the piston to wall clearance. If the stock bore is worn at all then they could well be too small and a total waste of money.

So out to 85.5 it will be for the new block and a set set of pistons. These ones have done a scary amount of work so time to retire them.