Outer Limits 4x4 Board

A couple of times now but months apart I have suffered a loss of power and the engine starts running very rough. Happens when the engine is hot. I put my foot down to accelerate revs climb to 4000K and no acceleration. Revs drop suddenly to 1500-2000K and it feels like the engine is about to stall. The engine has overheated a couple of times eg. boiling after the radiator got clogged with mud. No dash warning lights. Any ideas?

how hot is hot it can only take one time to overheat and crack a head

Hot enough for the temp gauge to hit max and hear the water boiling in the engine casing. It took about twenty minutes or so before the water stopped boiling. The problem only seems to occur when the engine is hot and been running for more than an hour or so and is really bad on steep hills. Its OK for short journeys. Some suggestions have been the fuel filter or the auto transmission bands however for the later the problem was evident in neutral and just reving the engine so unlikely to be that.

If you overheat a working engine you also overheat your auto.
If the engine is reving but no accelaration surely there must be slippage.
If you have a Roof rack, a Tow bar, Bigger tyres, do lots of Stop-start driving, or go Off road, you need a remote auto cooler, as big as possible and not piped through the rad.
Top up the cooling sytem to the filler neck and, when hot, look for bubles coming out of the cooling system with the rad cap off.
This can indicate compression loss through the cooling system.
J Top

Was told today I had two problems:-

Timing belt tensioner had disintegrated and the manifold had a crack in it. Getting the timing belt tensioner fixed but will leave the manifold for the time being.

your manifold will be why its running lean (revs up and down) you can try some "quick steel" on it for the time beeing...unless its under warenty

(Top up the cooling sytem to the filler neck and, when hot, look for bubles coming out of the cooling system with the rad cap off.)

Don't ever do this guys as the thermostat temp on late model vehicles is above boiling point at atmosperic pressure, so your going to get a lot of bubbles all right & 3rd degree burns to go as well.

The correct procedure is to remove thermostat for the test, fill and circulate water ensuring all air is purged then when engine is stone cold, check for bubbles whilst at idle. this eliminates false readings from any spot or micro boiling bubbles when engine is warm.

Also I beleive the cracked manifold would be an exhaust manifold (it's a common Mitsubishi fault) so this wont affect engine indle at all.

Are we having fun yet!!!

most thermostats run in the 84 to 92 degree region.
You can get a high flow 96 degree thermostat for racing applications but I have never heard of intentionally running the COOLING water at BOILING point in any automotive engine, pressurized system or not.
J Top

Well there you go you've learnt something new, a 92 degree thermostat does open at approx that 92, that only leaves 8 degrees at atmospheric pressure before coolant boils, the coolant at the back of the heads & around the exhaust ports can be at least 20 degrees hotter than that at the thermostat, this is called micro or spot boiling & it is very destructive to metal components in this area, so manufactures use pressurised radiator caps and additives in coolants to increase the temperature at which boiling will occur, this is just a small sample of how late model vehicles differ to one say 1o years ago, so when the back yard boys muck around with modern cars means more $$$$.00 for us repairers.
Cheers.

Ingenious-Eng wrote:
Also I beleive the cracked manifold would be an exhaust manifold (it's a common Mitsubishi fault) so this wont affect engine indle at all.

Are we having fun yet!!!

I disagree. Cracked exhaust manifolds allow fresh air to be drawn in (venturi effect) to the manifold, which then effects the exhaust gas oxygen sensor, causing the Engine management system to richen the fuel mix or to lean it out. The reason it would be happening mainly when the vehicle is hot is due to the expansion and contraction of the exhaust manifold.

My wifes toyota corolla had a cracked exhaust manifold (common to Toyota's also), and after having it repaired (a mate who is a professional welder, speciallising in cast), the car ran better, had more power, and returned an extra 100km's per tankful (not bad on a 43 litre tank).

Cheers, Dean

Tas_Dean You must of had a decent size crack or just in the right spot to get the venturi effect on your vehicle, as I know of heaps of Mistu's running around with cracked manifolds with no idle probs. Your theory on why it only happens hot may have some creed thou, as if it is sucking air the oxygen sensor is not used until the engine reaches operating temp also known as closed loop.
Cheers.

There is no way a crack in the manifold can allow air to enter the exhaust system. The gas is forced through the manifold by each cylinder. the only way air can get into the manifold from outside is fi there is a depression or vacuum in the exhaust manifold. Gas will be forced out of the crack, it won't draw it in.

Onlya crack in the inlet manifold will effect idle and power.

As for coolant boiling at 100 degrees, even with the radiator cap off it won't boil until at least 105, and under pressure (ie with the radiator cap on) 115 degrees C

Gas flow inside an exhaust system isn't that simple. Yes, it comes out of the cylinder under pressure - while the valve is open, and the gas is still burning. What happens when the valve closes? The gas has mass and velocity, and hence momentum, so it'll keep moving - what's coming in behind it?

Once the exhaust gas is outside the cylinder it cools down in a hurry - and contracts. There's nothing else in the pipe to make up for the reduced volume of the contracting gas, so the pressure must drop.

Tuned length extractors rely on a scavenging effect; an exhaust pulse exiting one runner helps draw the next pulse out of an adjacent runner.

A leaking exhaust gasket can also cause the exhaust to "crackle" on deceleration - that's not because gas is escaping through the leak.

At times, a leaking exhaust manifold will draw air in rather than blowing exhaust out.

Cheers,

Scott

Well covered Scott
J Top

Thanks for the moral support Scott!

Very well explained.

Cheers, Dean

Rossco wrote:As for coolant boiling at 100 degrees, even with the radiator cap off it won't boil until at least 105, and under pressure (ie with the radiator cap on) 115 degrees C

Your assuming at least 30% Ethal Glycol concentration is present in the coolant your basing your figures on, I was basing mine on coolant containing only inhibitors present as I very much doubted the guys using this information in this forum would own a Ethal Glycol concentration test kit, for the test as outlined above in relation to checking compression gasses in cooling system.

And I covered higher boiling points later in the paragraph, here's a copy

(so manufactures use pressurised radiator caps and additives in coolants to increase the temperature at which boiling will occur)
Cheers

Thanks for the input guys.
The consensus seems that it is the manifold causing the problem. I have checked my fuel records and recon it has had a crack for the last 20,000K including a trip up to Darwin and another through the High Country. Its now really bad so I'm going to try and fix it myself but its a mongrel to get at and is under a heat shield.

Passenger side manifold was cracked most of the way round between the 2nd and third outlets. Not the sort of thing you would expect at 110,000KM and 3.5 years. Tried to repair it with thermo steel but it just cracked again.

Unless your friends with a good cast iron welder, I'd be getting extractors!

When the corolla cracked the manifold, I got a couple of prices, and extractors FITTED were half the price of a replacement manifold Supply Only! And it is apparantly a common problem, so I could not see the sense in replacing a problem product with another problem waiting to happen, but as it turned out I got my manifold welded and resurfaced for a good price!

IF you don't want extractors, ring around the exhaust shops, they have probably got some manifolds they have taken off to fit extractors!

Cheers, Dean