Clogged Catalytic Converters??

Brenda ( snipped-for-privacy@hotmail.com) wrote on Thursday 02 September 2004 02:13 pm:

Brenda,

I just replaced mine. The $220 is an OK price (I found one place for $205 each), but they're not Nissan parts:

- No insulation around the pipes

- No mounting point for the bottom heat shield

- No threads for the studs connecting the catalytic converter to the exhaust

- No threads for the air pipe brackets

- Will probably require some bending of support brackets

In addition, no gaskets are included. Six are needed (four of one type, two of another), because you'll have to drop the exhaust crossover tube to remove the catalytic converters. Also, you may have to replace the air inlet tubes, which will run you about $50 each.

That being said, the actual labor should be two hours or less. If you have all the parts, removing and re-installing the catalytic converters is not difficult at all.

No, I've never had to replace them on any of these models of Zcars. Is the car o-heating?

BTW on this car the cats are further back, not right at the back of the engine. If the cats are overheating, that is a sign of other problems like the car running rich, not "clogged converters". The early converter cars (like 70's american cars) were bad about this, these later cars almost never "clog" converters. Backyard mechanics recall their Buick clogging a converter so still toss this term around. Like "rattling valves", it's not what's actually happening.

Depends on the area. In Atlanta it's $75 an hour and up.

Then do you have any clue what it could be? The car is not technically overheating --- I have a year old radiator and the heat is not coming from there. It's coming more like from the space just to the right of the battery although the whole front is very hot (not really the whole engine --- that is just hot like a Z is normally hot) but I am getting extreme heat from that region. So much that I am afraid to drive the car --- after a 3 mile drive it will heat up that much and when I pull it into my garage, I smell what I think is plastic burning or some kind of burning smell (not a rotten egg smell). I have owned this car from day one --- I know this car and I know something is terribly wrong. I have been to three mechanics --- one said it was the alternator. The second said the alternator was fine and nothing was wrong with the car. The third said I was leaking a little oil from a bad oil sensor and it was falling onto the exhaust and smoking and he replaced the sensor and degreased the engine. I drove home and it was so hot that I was really alarmed. I don't have a lot of time to mess with mechanics so if can't get a handle on this pretty quick, the old boy will be up for auction on EBay soon. Any ideas??? Thanks.

Read further down in my post, I explained that if it's running rich it will cause the converters to o-heat. Most likely cause is dead O2 sensors.

Have you looked under the car to make sure a trash bag or something else plastic isn't stuck to the exhaust? Very common thing to happen and smells awful.

He's an idiot, avoid him in the future.

That sound likely.

How did you know "it was so hot" driving it? You're going to smell some "burning" after someone cleans up an oily mess.

Yea a new car is much cheaper than getting this fixed, I'd sell it and go buy a new Z. That's only what $30,000? :-)

Have someone check the CO/HC before the cats and see what it's putting out. If the cats are really geting this hot, it probably is running rich.

Steve, your insights are terribly helpful --- the problem of course on a car this old is when I reach the point where it's going to be one big repair after another --- I want to cut my losses before I get to that point. So far, it's been a great car and the last few years especially has been very economical and needed almost no big repairs (seems I replace the oil pressure switch a lot! and it did need a new radiator). I have looked for the plastic bag stuck to the bottom --- no sight of one. What is an O2 sensor? some gadget that decides what the oxygen/fuel ratio will be?? And what is the "CO/HC before the cats"? Forgive my ignorance, I don't know much about the mechanics of a car and I want to be able to explain it to mechanic #3 (as you can tell, I had the most confidence in him and followed his suggestions

--- the first two did not impress me and I just left). After I got the car back from Mechanic #3, I drove a couple of miles to work --- smelled like cleaner burning off so I didn't fret. Drove the car home

3 miles from the office --- hmmmm, smelled cleaner and maybe a hint of plastic in the garage. Next day, drove three miles to work --- smelled burning and got strong sensation of heat radiating from same place as before. Drove home three miles from work --- strong smell of burning, opened hood and could feel a lot of heat from just in front of passenger compartment. Like I said, I know my car and I know how it should be. The "running rich" scenario sounds plausible to me since I felt it was running rich a while back (could smell some gas sometimes on start up). (Haven't done anything the last few days since we are all shut down and inside waiting on Frances --- I'm in SoFla --- if we get major flooding I won't be worrying about the Z!!). If it is the O2 sensor --- what kind of repair bill am I looking at? And will the cats be okay??? Anything else you would suggest I mention to the mechanic?? Thanks so much --- Brenda.

Like John smith and Steve T said the cats shouldn't give any problems unless the mixture is rich for some reason the O2 sensors are a good possibility have "mechanic #3" check your computer codes and see if there is anything indicated. As John Smith said if the cats are plugged they probably gat hot enough to melt the insides of them an indication of a mixture problem.

O2 sensors are what tell the ECU the fuel mixture. Most people recomend changing them at 60K intervals and yours has 3 times that so it's safe to say they need replacing. A NA 300ZX isn't too hard to change, call your "good" mechanic and ask for a price. Or take it back by and tell him what you just wrote here. I wouldn't say "It's still doing the same thing", that just irritates mechanics! :-)

Tell him someone said the O2 sensors might be making it rich causing the cats to run hot and see what he thinks. Or take it by when they ARE hot and show him. I think he did fix a problem (the obvious one?) this sounds odd what you are describing. I have no idea of the service history or if someone has installed the wrong plugs (misfiring can cause cats to run hot) etc so this looking at your car over the net is just a guessing game.

Power has been out since Saturday night from the hurricane -- had a very tough four days in 90+ heat with no A/C and no car. But the bright side is no flooding!! Doubt my mechanic will have his power on till tomorrow or next week. I've done most of the recommended maintenance on the car --- the last big job was the stuff done at 120K (I think I had it done at

117K) and I had it done at a Nissan dealer. If they change the O2 sensors then as part of a tune up and things they do then, then they were changed but looking at my receipt, I don't see a specific reference to O2 sensors. As I look back over the last year, I notice a couple of things that I kind of discounted at the time but that may be pertinent. In addition to occasionally smelling fuel at start up: 1) about ten months ago, I started to lose gas mileage --- fairly significant drop

--- always got at least 300 miles to a tank and suddenly started getting about 250 --- I just chalked it up to moving to Florida and using the a/c more often; 2) when I step on the gas, sometimes it feels sluggish like something is clogged and the gas doesn't go through --- I have to press harder on the gas pedal to go; 3) car idles rough on start up although when it warms up it smoothes out and purrs. Before I moved to FLA, I lived in LA and the car always passed all the smog tests with flying colors. Is there anything else to check that could cause the car to run "rich"? is there a fuel pressure regulator? I had a bad fuel injector once and it was so blatant --- the car drove very badly --- that I don't think that's the problem. Does my car have a computer? Should I ask the mechanic if he has an appropriate machine to test it? Again, everyone, thanks for your help --- I like my car but right now, it doesn't feel safe to drive and I need help talking with the mechanic.

Not a service item. Neither are cam belts. If you don't ask they don't do them. It's also down to you to know which service it needs. Lots of cars never get a coolant or brake fluid change as people just book in for an annual service and not the 2 year service (usually not willing to pay for it).

Yes. It also has a hot start solenoid. This switches the FPR from engine manifold pressure to atmospheric pressure. Supposed to only operate when engine is started warm and then only for 3 min but there are cases of it failing in hot start mode. A poor or bad temperature sensor and/or sensor wiring is the prime cause. It's also common to just bypass it and connect the FPR direct to the port on the plenum. In very hot climates this could cause poor hot starting.

Yes

While he has the O2 sensors out get him to put a pressure gauge on the exhaust. If the cats are plugged it will read high. Or a simple test with a vacuum gauge on the inlet plenum can also tell a lot about the engine condition. Plugged exhaust as in case 14 in the link below. Get a boost gauge, it reads vacuum too.

Peter and everyone, thanks for all your help. I'll take some notes when I go to see the mechanic (much of what you say is beyond me -- inlet plenum?? sounds like a body part). At the moment, fixing the fabulous Z-mobile will have to wait till at least next week. Ivan is breathing down our necks and I can't take anymore so am flying north tomorrow for at least a week to ride it out. If the Z is still here when I get back, then I'll work on getting the old boy up to speed. To keep this note on topic: today, I went down to start the car up and let it run a bit since it will be sitting another week. As always, it roared to life right away and did idle a bit rough though it smoothed out when I put it in park (it's an automatic). I let it run just a few minutes, and when I shut it down, I opened the hood to see if I felt excessive heat --- no heat but very much smelled fuel and it seemed to be coming from the space just to the right of the battery at the front of passenger compartment, the same space that seems to radiate heat when I drive very far. If the heat is not coming from the cats --- could something else cause it? Could I have a gas leak there? I don't see anything on the garage floor. Could the gas drip onto the exhaust and cause heat? The good news is that the work that mechanic #3 did --- changing the oil pressure sender -- does seem to have stopped all oil leakage since the piece of cardboard I put under to test last week was dry today. I think the Z --- being as old as it is --- probably has some seepage but so far, it doesn't leak enough so I ever have to put oil in between changes. Now, I just have to get to the bottom of the excessive heat

--- I won't drive the car if it is unsafe and this feels unsafe, like the car will burst into flames. Thanks again --- and I'll keep you posted about what the mechanic says when I get back. Brenda.

Yes cam belts ARE a service item, every 60K they should be done. Read the manual. They normally don't replace O2 sensors unless there is a problem but their life is estimated at 60-90K and still be responcive.

On the gas smell, the fuel hoses on the top of the engine are bad about leaking when the car is cold. Not really a fire hazzard from what I've seen but should be looked at. I can relate to the no power thing! :-)

Damn, you're good! Dropped the Zmobile off on Saturday with Mechanic #3 -- spoke to him today and he says the only thing the computer shows is a bad O2 sensor on the driver's side --- he ordered the part (from Nissan --- he says we can't use other parts for this??) and will have it ready tomorrow. I hope that cures the heat problem and I can get a few more miles out of the old boy. Bill will be about $270 inc. tax

--- does that sound close to reasonable? I like this mechanic and even if he charges a few bucks more than the cheapest place, I would pay it as long as he is in the ballpark. Thanks again so much for your help --- and I took your hint on talking to the mechanic --- told him I was happy we had stopped the oil leak but thought the car was too hot and told him about this group and just "suggested" that perhaps a few of the things you all had mentioned could be checked out. He seems like a pretty nice guy and a competent mechanic --- worth his weight in gold. Again, I am mighty impressed at your "diagnosis" --- if you're a mechanic, you must be fab. P.S. I went to college at a little school in Atlanta called Oglethorpe U. on Peachtree not too far from Buckhead. Atlanta's a great town --- and judging from the stomping they gave the Rams, the Falcons might be a decent team this year!

Smart guy, why use cheapo parts?

Yep, for a good mechanic it sounds reasonable.

Bingo. The cheap guy whould have installed so BS cheapo converters for more money and not fixed the problem!

That's the way to do it. They are human and insulting them isn't going to get you anywhere.

Yep and thanx!

I live about 5 minutes from there...

Steve, we fixed a problem but not the problem --- last night, drove home from work (about a 3.5 mile drive) and again, parked in my garage and felt extreme heat and smelled burning plastic --- is this a wiring problem?? This thread is getting too long so I'm going to start a new one

--- if you would, put on your "diagnose" hat once again and see if you can come with any possibilities. Thanks. Brenda.