Need Help with a Rough Idle/Coolant Loss Problem

98mustang3.8L

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Mar 9, 2018
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Hey everyone! I'm having an issue (or maybe 2 separate issues) with my 98 that I can't seem to figure out. Sorry for the long post, but I'm going to try and explain this the best I can.The car has about 200,000 miles on it and it's stock. I've had slowly disappearing coolant for a while (no signs of a leak that I could tell) and it hasn't really worried me that much. I check the level frequently and keep it filled and I haven't had any overheating issues. I figured that the water pump was probably starting to weep, since it is the OEM pump. Within the last few weeks, it's gotten worse and I can smell coolant now when I get out of the car. I started looking around and can't find any coolant leaking from the pump. I did notice one drop of coolant around what I think is the lower intake manifold where it meets the block. It looks like there is a rubber gasket there that could potentially be leaking.

I'm not sure if this is related but the car also idles rough when restarting when the engine is hot. Today, I drove probably about 12 miles into town and stopped for gas. It restarted fine after that. I went maybe another 10 miles or so and went in a store for about 20 minutes. When I tried to restart the car, it idled very rough and knocked off. When this happens, I have to put it in gear and go as soon as possible to keep it from knocking off. Then it kind of sputters and stumbles while I drive out of the parking lot. The more short trips I make once it's up to temperature the worse it is. But, once I get moving, everything seems normal. It idles fine at stoplights and if I put the car in park and leave it running it's fine - the idle's not perfect but no where near bad enough to knock off.

I also have a check engine light. It had 2 codes P0171 and P0174. I did some research and someone suggested checking the fuel trims to see what was going on. The long term fuel trims are way off - like +25%. The short term fuel trims are within +- 5% at idle and go up to 20-25% when the engine is under load. I read somewhere that if the fuel trims are registering like this, it could be an EGR/DPFE sensor issue. I cleaned the MAF sensor and also replaced the DPFE sensor (which did test bad) and that didn't help. Another possibility was a fuel delivery issue, but I would think that would cause problems all the time if that were the issue. I thought maybe if the intake manifold gasket was leaking, it could cause a vacuum leak? But, don't vacuum leaks show high fuel trims at idle and normal fuel trims under load? I don't like to just throw new parts on, without having a reason, but I'm running out of ideas.

Does anyone have any suggestions of things to test? Could these 2 issues be related? And if the lower intake manifold gaskets need replacing, is that really bad news for the engine?

Thanks for any help,
Rebecca
 
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wmburns

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Sorry for the long post, but I'm going to try and explain this the best I can.The car has about 200,000 miles on it and it's stock. I've had slowly disappearing coolant for a while (no signs of a leak that I could tell) and it hasn't really worried me that much. I check the level frequently and keep it filled and I haven't had any overheating issues.

Does anyone have any suggestions of things to test? Could these 2 issues be related? And if the lower intake manifold gaskets need replacing, is that really bad news for the engine?
Given:
  • the age of the car.
  • the high mileage of the car
  • The V6 from this time period is well know for head gasket problems.
  • The fact the motor is eating coolant and you don't really know where it's going.
  • The fact the motor run poorly.
If this were my car I would want to know what I was working with. The first test I would recommend is a compression test and a cylinder leak down test. Then you will know the basic health of the motor.

IMO it's also likely that there are other vacuum leaks. But should new head gaskets be necessary, repairing an intake leak is an added "free bee" of the head gasket job.

Want more proof of where the coolant is going? Pull the spark plugs.

Even more proof? They make a kit to test the coolant for combustion byproducts.

It's not a great idea to ignore coolant leaks especially ones likely going through the combustion chamber. The coolant will foul the spark plugs and ruin the cats. Spark plugs are cheap. Cats are anything but cheap.
 

98mustang3.8L

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Mar 9, 2018
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Thanks for the feedback. I finally got a chance to do some looking around today and I think I found the problem. I started off cleaning the MAF and I checked for vacuum leaks using the soapy water method. I sprayed around all the hoses, but didn't get any bubbles. There were one or two hoses that didn't look good so I'll probably replace them anyway. As I was looking around I noticed coolant where I think the lower intake manifold meets the cylinder head. I don't have any evidence that the coolant is mixing with the oil (no milkshake look and the oil doesn't smell like coolant). I plan on doing a compression test once I can my hands on a gauge, but I'm leaning towards bad lower intake manifold gaskets. I'll try to get a picture up to show where it was leaking.
 

98mustang3.8L

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Mar 9, 2018
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Here the picture I took of the leak. This is the driver's side of the engine. The bolt in the lower left corner is the thermostat housing bolt and the valve cover is to the right in the picture. You can see the coolant just under the spark plug wires where the manifold meets the cylinder head. I though about trying to re-torque the manifold bolts to see if that would stop the leak and fix the idling issue, but I guess if it's already leaking, the gasket is probably too far gone. From what I've read online it's a pretty common issue on these cars. Any tips on replacing these gaskets?

IMG_3015.JPG
 
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