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4.9 over heating - head gasket ? Sealers ?

3867 Views 42 Replies 10 Participants Last post by  dchavezo
I want to start by saying thanks for all help. I have a 95 Deville 4.9 that recently ran hot, I have changed the thermostat, bypassed the heater core, and had radiator professionally cleaned. temp still rising fast after temp hits about 80. I was told it could be a bad temp switch. but I have driven the car. temp hits as high as 250 but drops down to as low as 223 but seems to run fine. what could be the source of my problems. ps. once temp hit the 250 mark I parked it, I have never tried to drive it with it that high.
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Re: 4.9 over heating

If it read hot, but is not really overheating, it could be a bad ECT sensor. If it is really overheating it would be overflowing via the reservoir tank overflow line. If that's the case, check to be sure that the radiator cap is holding pressure.
Re: 4.9 over heating

When it is hot, open the hood to see if the fans are running. It sounds to me like ti is running with no cooling fans..
Re: 4.9 over heating

yes its not overheating. the temp reads high but the car never cuts off even at temps in the 250 range I can pop the hood and touch the engine with any problems


fans kick in at a temp of 225
Re: 4.9 over heating

when the temp gets high I have no heat and my reservoir fills up
Re: 4.9 over heating

Sounds like you have air in the cooling system. If it's not the cap, then you might have a HG failure. Time for a block test.
Re: 4.9 over heating

I replaced the cap same problem. but the car will idle for over an hour with heat running and temp will not get over 226. I am confused, I thought that with a HG problem it would run hot moving or standing still
Re: 4.9 over heating

If you JUST replaced the cap, give it a day or two to purge any air from the system. It will take the 4.9 longer than the 4.6. If you replaced the cap a while ago, then just check to be sure that the radiator is full to the top (cold) and the reservoir has coolant to the cold line.
I believe it ma be a clog in the cooling system. after the temp gets to about 250 one of the heater core lines gets cold but if I slow down and keep a steady speed of about 35mph the temp drops and I get heat.


Has any one here used HG repair in a bottle like Dura-Seal or K&W, if so witch one and what was the out come? I may have a HG problem on a 4.9 in a 95 Deville. but need the car daily. all replies welcome
I would recommend repairing the head gasket properly rather than trying a potentially damaging snake oil sealer.

You say "may have .... gasket problem" - but have you done a cylinder block exhaust gas test to confirm or deny that ?

Rent the test set from any large parts store.
:yeah: Don't waste your money on hope-in-a-bottle.
My Deville will go up around 250 while driving and back down again, nothing overly unusual about that. I forget if there is a valve in the heater line, but the only way for one line to be hot and other cold is if that's closed or the heater core is blocked, but you also said you bypassed it.
250 is HOT. It should never get that high unless there is a problem.
250F does sound hot. Mine usually stays between 194-212F, but will occasionally spike to 230F (in 95+ weather & heavy city driving).
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well I decided to replace the head gaskets, and found that the K&W head gasket sealer had clogged up the all the holes in my head gaskets. So I brought a FEL PRO gasket set from Rock Auto at good price and went to work on the car. sent my heads off to a local machine shop to get cleaned and resurfaced. but the question I have now is what is the fuel injector spec for a 95 Deville 4.9 my injectors are reading 11ohm and below? thanks in advance
Thats pretty low. I think the spec is 16, id replace them
I just want too tell everyone that helped me diagnose my problem thanks. it was warped heads. I broke the car down in the back yard and went to work on it, had the heads redone, picked up new injectors. reassembled the car and all is fine. thanks for all the help.
A mechanic friend told me a head job on my car runs about $2500 on average. My car is the 4.9. Do you guys think he may have been mistaken, and thinking of the N*? $2500 seems insanely high for an old-school kind of motor like the 4.9.
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