Piston Slap: Fix My Bro-Ham, Sanjeev!

Sajeev Mehta
by Sajeev Mehta
piston slap fix my bro ham sanjeev

Mark writes:

Hello Sanjeev,

I have a problem and hope you can help me. My Cadillac Brougham with the 307 V8 smells like gas under the hood. This is intermittent and the last time it was in the shop the mechanic found no leaks under the car or around the carb.

I did some internet searching and have heard all kinds of things including I probably used the wrong kind of gas. Apparently cars like mine can’t burn premium fuel completely and there might be residual gas left in the engine. My other cars use premium so I could have pumped 91 octane by mistake. Could that be it?

If it was a leak why wouldn’t it smell all the time?

I’m frustrated to the point where chancing it is an option so let me ask you this if you can’t fix it… if it is a small leak what’s the worst that can happen? I mean doesn’t modern reformulated gasoline have such a high flash point that I needn’t worry, except for the smell? Gas smell doesn’t really bother me.

If I took a fire extinguisher around with me could I “catch” a small fire under the hood in time to avoid damaging my paint? Are there warning signs, like smoke, before flames start to actually melt things? Does fire extinguisher residue clean up pretty easily?

Many thanks,


SANJEEV answers:

Mark: I’m searching for a clever–yet benign–way to spell your name wrong, but I got nothing. Plus, you got a machine that’s right up my Super Classy Alley, so I’ll proudly bestow my Sajeev Magic** on that sweet, sweet ‘Lac.

Old cars do stupid things because they are…wait for it…old. And you are freaking out with eleventy billion superfluous questions because of it: Fire extinguisher residue concerns? Really???

Stay calm: it’s all good, son! Leaks happen anywhere with old rubber and gaskets, especially with today’s ethanol-blend fuel added to the mix. (Literally.) If your carb’s never been rebuilt from the ground up, now’s the time. I betcha an internal seal is leaking, pouring fuel down the motor’s throat when it isn’t required. Perhaps it’s when the motor is cooling down (adding space between the seal’s gaps) and when the bowl is at a certain fill level. Or not. But whatever the internal fail, it’s only gonna get worse from here.

I had the same problem on an older EFI car, the fuel injectors were leaking internally and the smell was horrid. You can’t see an internal leak, but you sure-as-shit can smell it. So let’s address everything. Are there any rubber fuel lines under the hood? Replace them now, they are cheap too. Did ya install a fancy external glass fuel filter with a removable cartridge? Throw it away and get a conventional sealed filter. Don’t know a good carburetor tech in your area? Look harder, because now is the time.

About your Premium fuel problem, yes you are wrong for using it, but only your checking account is pissed at you. Premium fuel won’t damage an engine or leave unburned deposits above and significantly beyond a normal used motor. If you’re really concerned, you can run Seafoam in the intake and fuel system followed by an Italian Tune Up to really clean things out. After you have someone blow apart the carb and rebuild it.

**Patent pending. Or not.

Send your queries to sajeev@thetruthaboutcars.com. Spare no details and ask for a speedy resolution if you’re in a hurry…but be realistic, and use your make/model specific forums instead of TTAC for more timely advice.

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2 of 38 comments
  • Halftruth Halftruth on May 01, 2013

    I had this same issue on a 97 Chrysler LHS (I know, fuel injected vs carb'd but hear me out). Intermittent fuel smell that was strong when it would occur. I thought it could be the evap can as well so I replaced it as I had a spare. Nothing.. Same problem. So it got the point that I decided to get under the car and start yanking on the vent hoses that ran underneath the car. There were no leaks so I knew it had to be a vent. Wouldn't ya know. There was a disconnected vent hose that went from rubber to plastic. It had dried out and you could not quite see it as it was routed inside a rear frame assembly. I found out that the hose was actually a connector hose between a plastic and rubber hose. Replaced it and no more smell. HTH..

  • Psychoboy Psychoboy on May 23, 2013

    Late to the party, but I have to take slight exception to the "premium is cool for everyone" concept. My Fiancee's Scion TC hates premium during cold startup. Since high octane fuel is harder to light (which is the point), when it's just cold and humid enough, the motor doesn't want to light the mix and will stumble until it gets some revs together. Putting the 87 the owner's manual wants (it IS just a camry motor, after all), solves the problem. It might just be her car, but the result is the result. If the computer and the engine can't take advantage of the high octane gas, there's no real reason to use it.

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