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Old 05-22-2014
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Volvo md7a overheating, temperature fluctuating

Hello all, hoping to get some input on my latest volvo md7a woes. This is a 1979 MD7A raw water cooled volvo. Here is what is going on and I've tried

When I bought it the flowed almost no water and overheated.

1. pulled exhuast manifold, cleared blocked water passages, flushed out engine with hose.

After this it flowed a good amount of water but temp would creep up and would overheat if I ran it over 1500 rpm for any length of time. Going to lower rpm would bring the temp back down. Water felt cool/luke warm.

2. removed thermostat.

Ran cool, barely moving the temp needle off the peg. Water coming out the exhaust felt warmer

3. put in new thermostat and now it gets hot, just starts to overheat then the temp goes back down to 1/3 (well into the green) then back up etc etc. Sticking my hand in the water coming from the exhaust, it feels like the temp matches what the gauge says (very hot when it's near/just in the red, warm when it goes back down to 1/3)

Did I buy a bad thermostat? I don't have a thermometer to test it but I may have to go buy one. I cleared all the passages in the manifold and water flowed through everything I could find. The transmission leaks water at about 1 drop/ minute, but I don't think that's affecting this. Any experience with thermostats being bad from the factory? Other ideas?
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Old 05-22-2014
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Re: Volvo md7a overheating, temperature fluctuating

is water getting into your oil

fluctuating temps like that are indicative of a bad head gasket...but eliminate all possible sources before that as a last resort
water pumps good?
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Re: Volvo md7a overheating, temperature fluctuating

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Originally Posted by christian.hess View Post
is water getting into your oil

fluctuating temps like that are indicative of a bad head gasket...but eliminate all possible sources before that as a last resort
water pumps good?
Possible bad head gasket and no thermostat just masked that? There was no fluctuating with the old thermostat, just overheating at over 1500 rpm. Normal cruising is around 2000 rpm for reference.

The oil is dirty, but does not look milky to me.



I've seen oil where the head gasket has really gone and it's obvious, maybe this could be a kind of going but not gone thing? The engine starts easily and makes good power for what that's worth.

Water seems to flow well and the impeller is new this year.
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Re: Volvo md7a overheating, temperature fluctuating

definetely last resort

ok sooo

was your old thermostant oem?

is your new one oem?

thermostats are funky items...sometimes generic and cheap ones work just fine in good motors, sometimes they exhibit your symptoms

if the replacement is original there is a chance its defective,slight but still there

onlt way to check that is to test in hot water and make sure its opening at the temp its supposed to
if its not, or late, or sometimes its defective

get another one

since impeller is new and water flow is good look at thermostat for problems

FOR NOW! jajaja
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Re: Volvo md7a overheating, temperature fluctuating

btw oil looks might thin

what grade are you using?
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Re: Volvo md7a overheating, temperature fluctuating

Exhaust elbow may be partially blocked.
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Re: Volvo md7a overheating, temperature fluctuating

I know this might seem redundant but are you SURE you completely descaled and cleaned the manifold?, elbows? even hose?
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Re: Volvo md7a overheating, temperature fluctuating

even partially when in gear you can overheat...there are many passages and stuff that you cant get clean the first time

did you clean it in an acid bath or in a shop?
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Re: Volvo md7a overheating, temperature fluctuating

I did not do any sort of acid bath, just poked out the passages with a coat hanger. I'm thinking some sort of acid soak might be my next step. I just noticed a thread from about a month ago from a fellow with a MD6a with a similar issue that was resolved using an icemaker descaling solution.

The thermostat is a sierra aftermarket. No idea what the previous one was, it looked the same though. Testing it would make sense but for that to be worthwhile I'll have to get a thermomitor

Mixing elbow is clear, I checked that when the manifold was off.

Oil is supposed to be 20 or 30 weight, I haven't changed it yet.

Thanks for the replies.
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Old 05-23-2014
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Re: Volvo md7a overheating, temperature fluctuating

Quote:
Originally Posted by christian.hess View Post
btw oil looks might thin

what grade are you using?
My first thought, too. Looks mighty thin. Diesel fuel smell in oil? Might have a leak in the fuel pump. That would cause overheating.
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