Gasket Head cracks, then after repairs, catalytic converter problems? Connected or bad mechanic work?

[CajunSubOwner]

BLUF: Could failing/poorly functioning catalytic converter be related to/caused by previous gasket head cracks or by repair work done to replace cracked gasket head??

 

Mechanically challenged 08 Forester owner here. A few months ago, we found out that the gasket head was cracked. Right after we picked it up, we noticed that it seemed to have less power and, very obviously, the gas mileage was lower. The repair shop did not believe (or at least admit) that there was any connection between the replaced gasket head work and the substantial reduction (I calculated it to be about 3-5 MPG difference) in fuel efficiency. 

 

Three months after the repairs, the check engine lights came on. The repair shop advised that the sensors showed that the catalytic converter was failing/had failed. They said there was little difference in temperatures going in and going out of the catalytic converter system (I'm simplifying what they said but that sums it up). I asked about whether it could be caused by the engine not "burning" all of the gas, but they said that if that were the case the error codes would be different. And that I need a new catalytic converter.

 

I'm hesitant to shell out the cost without knowing why it failed (so that it doesn't fail again and I'm looking at buying another one. 

 

Is there a way to determine why the CC failed? Should I take it for a second opinion? What other questions should I ask?

 

Many thanks in advance for your help. 

 

 

 

[ccrinc]

If enough coolant from the bad head gaskets got into your cat, then yes, that could cause one to clog up and fail.  However, 3 months later is a stretch.  That should have happened much sooner.

 

I'd get a second opinion.  Have the shop put a vacuum gauge on your intake manifold.  (Yes, they're still around and still useful for diagnosing stuff a scanner can't.)  It should show 14" or better of mercury.  If it's below 10" (that's inches), it's likely you have a plugged cat which will have an effect on  how the engine runs and poor gas mileage.

 

Emily

[Fairtax4me]

This depends on the failure mode of the head gasket. Was it leaking oil or was it leaking coolant? Did the engine ever overheat? 

 

It's more common for the head gaskets on that engine to leak externally. 

Catalytic converter damage would only occur if the leak were internal, and the engine was burning coolant or oil. Internal head gasket leaks are NOT common on that engine, unless it is severely overheated. 

 

There are some things that could have been done incorrectly during the head gasket work that can result in lower fuel economy. And yes, over time a loss of fuel economy due to incorrect AF ratio, or incomplete burning of the fuel, will lead to failure of the catalytic converters. 

 

I agree with Emily, you need a second opinion on this. 

A 3-5 mpg difference in fuel economy is a big drop, and to me at least, is indicative of a problem caused by the head gasket replacement, especially if you noticed the difference immediately after the work was done.

Edited April 6, 2016 by Fairtax4me

[kamesama980]

<p>Either oil or coolant burning in the cylinder will cause the converter to fail. Either could happen due to a failed head gasket.</p>

<p> </p>

<p>Timing of the light (3 months later) might be slightly suspicious but not unreasonable. The catalyst code is a readyness test, not a constant monitor. That means you have to meet certain conditions and drive a certain way before the check will be run. going to school and working in a shop I've seen it happen and heard of it happening many times that some readyness test don't run for over a year. Particularly the cat test if you spend a lot of time in city stop-and-go driving. The prerequisites are usually along the lines of engine between A and B temp, vehicle speed between C and D, throttle not moving more than E for F seconds, no other codes, etc etc then it takes a few seconds to a few minutes to run the test during which conditions have to stay right or the test is aborted.</p>

<p> </p>

<p>That you took at 3-5mpg loss and noticeable power loss immediately after having the HG repaired is VERY suspicious. I would agree with other's suggestions of getting a different shop to look at it and see what might be up. The first thing that comes to my mind is if the cam timing is off a tooth some engines will still run but poorly, which in turn could cause the catalyst codes.</p>

Edited April 6, 2016 by kamesama980

[CajunSubOwner]

All:

 

MANY thanks for the assessments and the recommendations. I will follow up and re-post with update - probably in a few weeks.