isuzu turbo egt's

Hi Everyone,

I've just joined after searching google for information. Hopefully someone here can help. (Thanks in advance)

I have an old Bedford Commair bus and have been repowering with an Isuzu 6BG1 turbo.

It

John,

You will have to search for the allowable operational range of the EGT on that engine.

But as a matter of course the EGT will increase as you lean the engine out. On the flat in high gear you will have a pretty full throttle with higher fuel/air ratio - richer mix. But going up hills with a "light throttle" as you say you will be in a lower gear with high RPM. More air being pumped by the faster turbo but not as much fuel with the light throttle = leaner mixture.

A good part of the fuel flow on any combustion engine goes to cooling.

Not too many trucks of the older type have EGT gauges so nobody is aware of the constant changes. I would think what you are seeing is pretty normal but as I said, check the manuals or web to see where the red line is on the EGT gauge.

Really modern vehicles have computer mixture sensing and regulate the flows but they still get big rises and falls in the EGT according to the relationship between engine(turbo) speed and fuel flow from the right pedal.

Lang

Just found an Isuzu 6BG1 spec sheet on the web.

If fitted with a Garret TO4B83 Turbo EGT Max Normal Operating Temperature is 675 deg at the turbo exhaust inlet.

If fitted with an IHI RHC7E it is 750 deg.

Lang

Sorry Lang old mate, running a diesel lean will give low EGT's.
Extra fuel in a diesel makes for high EGT's.
(Exact opposite to a petrol engine)
But back to the question, those EGT's are OK.
450 is a fair number when working but I'm suspicious of the high reading when slowing down, is the gauge accurate?

Thanks everyone.

It's an IHI turbo, the pyro is a VDO gauge with an after market probe. I have an electronics background and am reasonably confident the gauge is ok.... but not ruling it out.

The probe is after the turbo and not before. Which makes me more concerned becuase everywhere I've read, you can drop a few hundred degrees across the turbo.

I've got the turbo off at the moment due to a cracked housing. I'm pretty sure it wasn't there originally and would (???) support the idea that it may be overheating or has.

Assuming it is, what should I be looking for?

Overfueling (?)
A slow spooling impeller leading to an effective exhaust blockage (?)

anything else?

It has a new radiator and not overheating water wise.

thanks

Jon.

Jeffo,

You are absolutely right, petrol and diesel are exactly opposite and my theory for the heating reason does not hold good in this case. Looks like Jon's truck is acting normally to load and increased throttle input.

Here is probably more information than you wanted Jon but if you can sit down and plow through it you should have a pretty good grasp of where the probes go and what they record in the various possible positions.

www.dieselpowermag.com/features/0607dp_e..._basics/viewall.html

Here is another very interesting explaination of engine temperatures petrol/diesel:

This varies for the engine design, and is also during the cycle and according to use e.g. running lean is hotter. For a particular petrol engine a figure for the average temperature in the cylinder during combustion stroke is 820

Be wary of any VDO gauge with a bit of age on it. After I left school, I harvested cane, using Austoft machines, for few years and quite often at night, the harvester driver would turn all his lights off, as well as the a/c and radio, and would spark up his lighter to read the fuel gauge. I can attest to it being quite a feat to harvest with no lights. We found that the more electrical load you had the more erroneous the reading, an overheated Powerhaul could be fixed by turning the flashing light off. This phenomenon only seemed to happen with VDO gauges though.

Dieseldog,

Would that gauge problem be more of a function of low voltage caused by too many things sucking off the one power source eg the back of the ammeter or ignition switch.

Do you think if you had a clean run to the gauge either with its own circuit or through its own relay it would have been OK?

Lang

The electrical are all good. The bus has been completely rewired. It's still 12volts, and the engine has been converted to 12volts. (12 volt starter, glows, alternator).

Lang,

I'm not sure, however all the Austoft/Case gear fitted with VDO gauges seemed to do it once they were a few years old. You could fit a new gauge and it would be fine for a while, but it ended up being cheaper in the long run to just turn all the lights out to get your readings and make sure you fuelled up after each 12 hours running time. We never had a problem with the the Murphy's, even though they ran from the same electrics. Even my dad complained about his Acco 2350 overheating on the way home every day, always seemed to happen after dark. Sure enough, turn the headlights off and she's back in the green. Also noticed that putting the ticker on at night makes the fuel gauge jump from half to three quarters and back again, in time with the ticker light.

DD