2003 Pontiac Grand Am V6 loses power up hills - catalytic converter?

A friend of mine has a 2003 Pontiac Grand Am V6 with about 250,000 miles which loses power going up long hills to the point that it can’t keep up speed. They took it to a mechanic, and from what they remember, he said that the catalytic converter needs to be replaced.

The SES light is on, and I will be putting a scanner on it sometime this week hopefully, but in the meantime, I have a few questions. My friend has already purchased another car, and I am trying to decide if it is worth fixing to use as a backup car for myself before they send it to the junkyard.

My first question is - would a plugged catalytic converter cause the symptoms described above? I can buy a brand name universal catalytic converter and have it installed for a total of about $175.00. If that takes care of it, it would definitely be worth it.

My second question is - assuming it is the catalytic converter, do they just get plugged after that many miles, or is there something else wrong that caused it (such as too rich of a fuel-air ratio)?

My last question for now is - the exhaust shop told me that I need to make sure that the catalytic converter is “OBDII compliant”, or the check engine light will come on. I know what OBDII is in general, but not as it relates to catalytic converters. How do I find out if it is “OBDII compliant” before I buy it?

Thank you!

One of the common causes of loss of power under load is a restricted exhaust - and one of the most common causes of that these days is a plugged converter. But there are other causes as well. Another common one that fits the symptoms is poor fuel pressure (plugged up filter or failing fuel pump, for example). There’s no need to guess. Do you know how was it diagnosed? A vacuum gauge is a simple way to check for exhaust restrictions, and a fuel pressure gauge for the fuel system.

At 250K miles, I don’t think it would be too surprising for the cat to get plugged, but yes it often comes from other problems as you suspect. Running too rich, burning a lot of oil, etc. Basically anything that dirties up the exhaust.

I will stand to be corrected on this, but if you go to any parts supplier and give them the vehicle info you will get an “OBDII compliant” converter.

Thank you for the reply. It was diagnosed by my friend’s mechanic, and my friend doesn’t remember anything about it except the diagnosis.

I’ll post more info once I pull the codes tomorrow and put a vacuum gauge on it. Hopefully I can find the correct vacuum specifications somewhere.

Do you have to be driving it for the vacuum test to be accurate (however that would work), or just rev the engine?

Here’s a simple guide to the vacuum gauge: http://www.gregsengine.com/using-a-vacuum-gauge.html

You just hook it up and idle the engine. Typical vacuum reading on a healthy engine at idle is somewhere between 17-22 inhg (steady reading). If you rev the engine up and hold it somewhere near about the 3000 range the needle should stabilize and remain about the same as idle vacuum. If you hold it revved and vacuum is lower or declining (or both) that indicates exhaust restriction. Of course, a more primitive DIY method is just to pull out the upstream O2 sensor(s) and test drive it. If there is an exhaust restriction the open hole from the sensor keeps the backpressure from building. Note that you CANNOT just drive around that way. It’s just for a quickie test.

I like Cig’s “quickie test”. I’ve never heard of that one before, but it makes sense.

Just as a bit of additional information, an “OBDII compliant” cat converter will have a bung hole for a downstream oxygen sensor to be installed. Prior to 1996 it was not required to monitor the performance of the converter. Since that model year, it has been mandatory, and that’s what the OBDII system does (among other things). It monitors the converter by comparing the oxygen level before entry of the exhaust with the oxygen level after the exhaust passes through the converter. It uses a second oxygen sensor placed after the converter to do this. It compares its signal with the oxygen sensor before the converter, the primary function of which is to help the computer meter the proper amount of fuel.

@cigroller, thank you for that link.

@the same mountainbike, that explanation is helpful. What I was getting confused about was that the universal catalytic converter that I was looking at did not have the additional length of pipe where the oxygen sensor would be installed, but rather the old converter would have to be cut out and the new one welded in.

My friend spoke with the mechanic some more today, and they told him that the car also has the famous 3.4L intake manifold gasket leak. Is there an easy way (i.e., that does not require engine disassembly) to tell if the coolant has leaked into the oil (I guess that’s the place it would go) as well as how much (if any) damage has been done to the engine from this? I don’t know how long it was leaking for.

I found what looks like a good write up here on replacing the intake manifold gaskets: http://www.d-tips.com/general/articles/article.aspx?id=2 Has anyone here done it, and how difficult is it for the average backyard mechanic?

Sincere thanks for the follow up. Let us know what you decide. We do care.

You can tell if the oil is seriously contaminated with coolant pretty well just by looking at it. It will get a milky appearance, and sometimes gloppy. It gets to look like a chocolate milk shake.

I haven’t done those gaskets though I looked at the procedure back when I owned a vehicle with one of those in it. It didn’t look difficult in any technical sense - just a fairly substantial teardown on the top end. If you weren’t in a hurry and has a good repair manual and appropriate tools (e.g. an actual quality torque wrench would be a must) I’m sure its doable by the somewhat experienced DIY mechanic.

I checked the car out in person today, and the only code it had was P0405 - EGR Flow Sensor A Circuit Low Input.

However, when I checked the coolant surge tank, the coolant looked like a rich, dark chocolate milkshake. I assume this is a head gasket issue, so I’m not sure I want to get into that repair on an engine with 250,000 miles.

Sounds like scrap to me too, although it’s only an '03 and the Grand Am has something of a following. Someone might want it for parts or to put back on the road for something more than the scrap price.

My friend was going to sell it to me for $300, but I’m not sure it’s worth me putting the time and money into.

Well, $300 is pretty close to scrap price, so if you picked it up you’d probably no lose much if you ended up not doing anything.

My own thought is that doing the heads and LIMs would be worth it (if most of the rest of the car is ok), but then you still have a transmission at 250K (I assume). But somewhere in the world is a wrecked Grand AM with a perfectly good engine/transmission in it. Perhaps you could thing about finding that and just dropping all of that in. If you have to pay a lot to get them and pay someone to install though, now you’re getting into “probably not worth it” territory.

@cigroller‌ I understand what you’re saying as far as the head and intake manifold gaskets, but my concern is the condition that the engine may be in from however long the coolant was mixing with the oil.

Also, I neglected to note that the oil in the car was rarely changed, and was 2 quarts low at least once. In addition, the car is not in great cosmetic shape (some dents, peeling dash, dirty carpets).

As far as dropping in a new engine and transmission, if I bought (or rented) an engine hoist, perhaps I would be able to find one in good condition to pull myself and install myself, but it wouldn’t be worth the cost if I had to pay for an engine/trans that was already pulled, and especially if I had to pay to have someone else put it in. In that case, I’d just pay someone to do my Northstar head gasket/stud job (although so far it’s holding up as it is).

My main hope when I started the thread was to be able to simply change the catalytic converter and have a car that wasn’t very pretty, but ran well enough that I could use it as a backup car when I worked on my current car. As it stands now that I was relayed the additional info, I don’t think it’s something I want to attempt to tackle.

Yeah, if its kind of ugly too then your friend should either scrap it or perhaps try to find a Grand Am enthusiast looking for a parts car or something.

They have been offered $300 by a junkyard.


Even without seeing the car, I’m 80% certain your headgaskets are fine

Why make things more complicated than they have to be?

Those engines were notorious for failed intake gaskets, NOT head gaskets

Once you replace those intake gaskets, I’m willing to bet you won’t have that nasty looking coolant and oil anymore

For $300 plus a few bucks more for the cat and intake gaskets, it’s worth it

@db4690‌ I know the intake gaskets are definitely bad, but do the intake gaskets cause it to run hot at times, even in the winter? I did leave that out of the details before.


If you’ve got bad intake gaskets that are allowing coolant to pour into the crankcase . . .

I’ve seen it before

@db4690‌ I didn’t check the oil, I checked the coolant (looked like a dark chocolate milkshake). Would the bad intake gaskets cause the oil to go into the coolant?