'91 GT weak cylinders

heetseeker91 · Feb 27, 2010

Ok i'm about ready to pull my hair out so i figured i would get some advice from my fellow mustang owners. I will first off admit that i am a mustang noob so i don't know much about what too look for. I have a '91 GT that aside from a cold air intake and exaust, it is stock. A few days ago i added 1/2 qt of oil and some coolant, drove the car home and parked it overnight. The next day i go to start it and it feels like its missing horribly. I take the car to a shop they say that they ran a compression check on the engine and that i have two weak cylinders and that was the problem and the only way to fix it would be to rebuild the entire engine. Can two cylinders go bad overnight? i mean when i drove it home it was running great. I have recently replaced the plugs and plug wires. Is there anything else that i can check before biting the bullet and having a rebuild done? I apologize ahead of time for my noobiness lol....

cjrpony · Feb 27, 2010

Have you been losing oil and coolant, is that why you added some? Sounds like a blown head gasket to me. That would explain the coolant loss and weak cylinders and if you are burning oil also, then yea, maybe you need to rebuild. And don't worry about the noob thing. Unless you were born in a Mustang, then we were all noobs once...

Mustang5L5 · Feb 27, 2010

Could potentially be a Head Gasket. Need more detective work though before I would rip off a head?

How mechanically inclined are you? A HG change is maybe $75 in gaskets and fluids if you do it yourself.

cjrpony · Feb 27, 2010

Did you check to see if you have coolant in your oil? If so, that is a sure sign of a blown head gasket. Do you know anyone with a compression tester? You could test the compression yourself if you don't trust the shop. Or you could take it somewhere else for a second opinion. How many miles are on this motor by the way?

Mustang5L5 · Feb 27, 2010

I wouldn't rebuild the entire engine though like the shop says.

cjrpony · Feb 27, 2010

Yes, I agree, if it is just a head gasket, do it yourself, or get a buddy to help you out. It's not really too bad a job if you have the tools and a little knowledge and should be under 100 bucks. But of course, make sure that is what the problem is first.

heetseeker91 · Feb 27, 2010

Thanks for all the replies. To be honest a head gasket swap is pretty intimidating, however i dont think i can make it run any worse haha. I think i will try that on my next days off. since i was low on oil and coolant that might be the best way to go. if anything it would tell me if the bottom end of the engine needs work as well. just out of curiousity is there any online guides to doing this? if not i'm pretty sure i can figure it out. Thanks again for all the input i will repost when i have the heads off and see what's really going on.

2000xp8 · Feb 27, 2010

Unless the car was mistreated, a blown head gasket on a stock engine is pretty rare. Sometimes it happens when the coolant is never changed, it becomes acidic and eats away at anything in it's path.

Check your oil and coolant to see if any of it has mixed with each other.
Also check the ground to make sure you have no coolant leaks.

Oh, and don't go pulling the heads off yet, IMO you need a second opinion.
Check the plugs to see what they look like too. A head gasket doesn't always cause a car to run that badly, sometimes it just eats water and oil, and half a quart really isn't that much to go by.

dgollem · Feb 27, 2010

I agree you need more diagnosis. Did the car run fine before your tune up? what was the compression in the "weak" cyclinders/good cylinders? How many miles on the engine? Did it over heat because it was low on water? Check the oil and water as stated. Could be you have a couple of weak cylinders but also a cracked new plug, bad plug wire or something else simple and related to the tune up.

Tanus · Feb 27, 2010

compression can be lost 3 ways; head gasket, piston rings or valve seats

to check rings you do a comp test, record numbers then squirt a little oil in cylinder, if numbers go higher its the rings

leaking from valves it'll usually have too much crankcase pressure, can pop out dipstick or pvc, or if you open oil cap smoke will blow out.

head gasket it might burn coolant (blow white smoke from exhaust) or get coolant in oil (oil will look like chocolate milk)

these are just a couple things for you to check, but whatever you do, Id stay away from that shop.

heetseeker91 · Feb 27, 2010

dgollem said:
I agree you need more diagnosis. Did the car run fine before your tune up? what was the compression in the "weak" cyclinders/good cylinders? How many miles on the engine? Did it over heat because it was low on water? Check the oil and water as stated. Could be you have a couple of weak cylinders but also a cracked new plug, bad plug wire or something else simple and related to the tune up.

yes, the car ran fine until i added the oil and the coolant. As far as what the compression acutally was i do not know because they didn't tell me. there is 118,000 miles on the engine. The previous owner, my uncle, did tell me that about 3 yrs ago it did overheat because of the water pump going out but it has never overheated since i've had the car, wish he would have told me that when i paid him for the car... lol. However i have done all of my own oil changes since i've taken the car and I have never seen anything in the oil. as far as the plugs and plug wires i have replaced them with new ones two days ago with no change to the situation. from what you guys have said on here it sounds like i need to get the tools i need and do my own compression test and find out if its the rings, valves, or still the head gasket.

2000xp8 · Feb 28, 2010

heetseeker91 said:
it sounds like i need to get the tools i need and do my own compression test and find out if its the rings, valves, or still the head gasket.

I agree, simply saying low compression in two cylinders doesn't really mean that much. You need to know for yourself.
Just because your car burns a little oil doesn't mean it's ready to go.
I drove on an engine that burned about 2 quarts between oil changes for about 3 years.

Now if the HG is blown, just buy an explorer 5.0 engine and swap it out, a stock engine is way too expensive to rebuild for what you get, and even if you do fix the HG, if you washed out the bearings, you will have just wasted a lot of time.
Explorer engines come with better parts and can be had for $500.

'91 GT weak cylinders

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