Dear Car Talk:
I accidentally put diesel in my gasoline engine. I drove about 50 miles before bad things began to happen.
Will this destroy my car?
— Sarah
No. This is a root canal, Sarah, not an extraction. This can be fixed — with some pain and money.
The reason it took 50 miles to start sputtering and conking out is because you still had a good amount of gasoline in the pump, the fuel line and the fuel rail. Once that pure gasoline was combusted, your engine was trying to combust diesel fuel, which it can’t do.
At this point, the diesel fuel has worked its way through your fuel system and into the cylinders. Not ideal. But I think the patient can be saved.
Your mechanic will have to clean out your entire fuel system. He’ll have to remove and flush out the fuel tank, the pump and the line. He’ll also have to remove the fuel rail — which feeds the injectors — and flush that, too.
There’s undoubtedly some diesel fuel inside the cylinders at this point, too. To get the bulk of that out, he can remove the spark plugs and crank the engine — blowing it out the spark plug holes. And once he gets the fuel injectors spraying pure gasoline again, running the engine should push the rest of the diesel out of the cylinders.
The other possible casualty is your catalytic converter. It’ll probably be OK, but it’s not designed to handle diesel fuel, so time will tell.
The biggest casualty, Sarah (other than your ego)? Your wallet. This is a time consuming, messy job. Removing the fuel rail, in particular, can be difficult on lots of cars. So, expect to spend many hundreds of dollars.
The good news is that the more you spend, the less likely you are to ever do this again. And I’m guessing this’ll be a once-in-a-lifetime experience for you, Sarah. So take pictures.
Dear Car Talk:
Can you recommend a good compression test gauge for a shade tree mechanic?
— Kevin
Interestingly, I can’t even remember the last time I used a compression tester, Kevin. So, if you can find mine, you can have it.
Cars just don’t burn through valves and piston rings like they used to. So, we rarely see cars with compression issues these days.
Plus, we do a lot of diagnosis using our scan tool. The scan tool can tell you, for instance, if there’s gasoline that’s not being combusted in one or more of the cylinders. But to find out why, you have to dig in and start taking things apart.
If you suspect that the problem is bad compression — due to a bad valve, a worn piston ring or a badly blown head gasket — a compression test will give you a definitive answer.
An engine works by compressing the air and fuel mixture inside the cylinder. If the compression doesn’t get high enough, the mixture won’t combust well, and the engine will run poorly — or not at all.
So, to test each cylinder’s compression, you remove the spark plugs and thread the compression gauge into one of the spark plug holes. Then you crank the engine, as if you were starting it, and check the reading. And you do that for each cylinder.
A compression gauge doesn’t have to be a precision instrument, because what you’re really looking for is a big difference between the cylinders. So if three cylinders are at 150 pounds and one is at 80 pounds, you know there’s a problem. If the absolute numbers are off a bit, it hardly matters.
If you go online, or to your local auto parts store, Kevin, you should see a wide selection of compression gauges for sale. You really don’t need to spend more than $30 or $40 to get one that’ll do the job.
Look for one with a hose, which makes it a little more versatile and easier to use. And then, once you’ve checked your own compression, your wife’s, and your father-in-law’s, maybe you can put up a yard sign that says “free compression tests” and make some new friends.
