Red-hot catalytic converter might be due to valve trouble (2024)

Dear Tom and Ray:
I have a 1998 Honda CR-V with about 189,000 miles on it. A month or two ago, the Check Engine light came on, and a scan said I had two bad oxygen sensors. I had them changed. Then, a few weeks ago, the car started stalling while driving. The Check Engine light came on again, and this time the scan said "Catalyst Efficiency Below Threshold." No problem. I had the catalytic converter changed. The car still stalled. I changed the ignition coil, distributor cap/rotor, spark plugs and wires, and cleaned out the idle air control valve. When I pulled out the spark plugs, they looked normal. The car still stalled. It seemed like it was the worst going up a hill around 60 mph, and it seemed like it got worse if the air conditioner was on. I took it to the dealership, and they tried telling me that the catalytic converter that was put on was too small. So I carried it back to the muffler shop. They told me there's no way it's the wrong size, but they have great customer service, so they'd do it anyway. Before he pulled the "old" one off, he said he drove the car and noticed that the converter was getting cherry-red hot. When he pulled out the old catalytic converter (it had been on only around 10 days), he said it had been melted! Something is running so hot that it's melting the catalytic converter in less than two weeks! I know it's losing some oil, but other than that and the stalling, the car runs great. It starts up every time, and it runs smoothly. I don't really think that gas mileage has been affected either. When the car stalls, we have to wait anywhere from five to 30 minutes before it will start again. Sometimes, it won't go 25 miles without stalling, but this past weekend, it went 75 miles before it stalled. I'm out of ideas; I need help! Thanks in advance, you guys! – Amhad
RAY: Gee, I can see you're going to be a hard guy to get rid of, Amhad. You've already tried everything. What did you leave for us to suggest? Acupuncture?
TOM: My first thought when reading your letter was that you had a plugged fuel filter. But then you threw in the red-hot converter thing, so that's not a fuel-filter problem.
RAY: If your converter is turning cherry red, that's because there's unburned fuel getting into the exhaust system and being combusted inside the converter. The question is: Why?
TOM: My guess would be that it's a valve problem. These older CR-V's tend to suffer from valve seat recession. So you need to talk to Janet Yellen about this, Amhad.
RAY: If the exhaust valves aren't adjusted regularly on this car (we recommend checking them every 30,000 miles on these older CR-V's), they can get too tight and burn out, or damage the valve seats. The valve seats are where the valves are supposed to nest when they're closed and make a seal. If the valves aren't making a tight seal, for whatever reason, unburned fuel can escape out of the cylinders and be sent down the exhaust pipe.
TOM: That also could explain the stalling, because bad valves give you bad compression. If too much fuel and air is escaping the cylinders rather than combusting inside them, the engine won't run.
RAY: When the engine gets hot, parts expand. The valves probably are expanding in the heat, to the point where they no longer seat well. That's why the car stalls. When you let them cool off for 25 minutes or whatever, compression returns and you're able to run the car again.
TOM: So you can start by asking for a valve adjustment. But if the valves or seats are already damaged, or a valve is already burned out, no amount of adjustment will help, and you'll be in for a valve job, Amhad.
RAY: Unfortunately, that's going to cost you over $1,000. But look on the bright side: At least the guys in the muffler shop won't all lock themselves in the men's room every time they see you coming.

Rolling wheels can jump-start diesel - in theory

Dear Tom and Ray:

I have a diesel engine. If I turn off the car while it is moving, then engage fourth or fifth gear and pop the clutch, will the engine jump-start? – Bobby

TOM: Probably.

RAY: As long as you’re moving fast enough.

TOM: Diesel engines don’t use spark plugs. They use high compression in the cylinders to create enough heat in there to combust the air and diesel fuel.

RAY: So then all you need is fuel delivery -- which you have when the key is in the run position -- and something to get the engine turning.

TOM: That’s what the jump-start is for. Normally, when you put the car in gear (which connects the engine to the wheels), it’s because the engine is already turning, and you want to use it to turn the wheels.

RAY: A jump-start (or roll start, most accurately) turns that equation around. Your wheels are already turning, and you want to use them to turn the engine.

TOM: So it should work, Bobby. The exact speed and exact gear combination will be different for different vehicles. I mean, if you have a big V-8 diesel engine with really high compression (which means it takes more force to make the engine turn), and you try to start it in too high a gear or at too low a speed, the engine could actually win that battle with the wheels and bring the wheels to a stop.

RAY: So, for instance, if you’re rolling at 15 mph in a big, honkin’ 6-liter diesel V-8, and you pop the clutch in fifth gear, the car may come to a halt, without starting the engine. So you would have to experiment with different speeds and gears to figure out where the limits are.

TOM: When you do that, wear your seat belt, a crash helmet and a mouth guard.

RAY: So theoretically, yes, you certainly can roll-start a diesel engine using fourth or fifth gear. But keep in mind that we do have a recent invention that makes this completely unnecessary in most cases, Bobby. It’s called the ignition key.