What happens if you put unleaded petrol in a car that’s supposed to take leaded fuel? Will there be an amusing amount of smoke billowing from the hood, will it just not start or will the car work fine?
I ask because my housemate told me he didn’t know whether his car took leaded or unleaded fuel, so he fills the tank with unleaded because he thinks it’s better for the environment. I’m trying to figure out if there’s going to be any interesting drama in the near future. 
The car should run fine. My 1964 Rambler runs great on unleaded.
On older cars (prior to 1970 or so) there is a chance that after a while, the valve seats will wear badly, because the lead was used to lubricate things. However, I understand that it’s possible to redo the valve seats with hardened inserts to eliminate this problem.
IANAM (mechanic) but I believe that leaded fuel coats the valve seats preventing wear.
Vehicles designed for unleaded have hardened valve seats so if the car was not designed for unleaded then expect increased wear on valve seats.
Because Octane rating of unleaded is lower I think timing may be out too (unless using high octane UL)
Don’t think it will do anything spectacular immediatley.
Cars designed for unleaded run fine on leaded (it was even recommended that every 3/4 tank should be leaded at one point)
Never put leaded petrol in a car with a catalytic convertor.
I was in the same situation when the sale of leaded gasoline was banned here in the early 1990s. At the time, I was driving a 1986 Honda Civic that was designed for leaded gasoline.
I asked a friend who was a qualified mechanic about this. He said it was OK to run unleaded in a designed-for-leaded engine (though he also mentioned the valve problem), but one should never run leaded in an engine designed for unleaded fuel.
My Honda ran just fine on unleaded gas, by the way, and I never had a problem. I sold it before the valves would have been a concern though. Anyway, your housemate should be fine.
There are some products available that minimise the damage inflicted on valves & valve seats by unleaded fuel. Here is one:
http://homepage.ntlworld.com/cedric.norman/classicbikes/cb_lead_replacement_petroleum.htm
And I’m pretty sure that there is an STP additive available, saw one a few years ago.
What they said. It’ll ruin the valve seats eventually, if it’s the Unleaded in Leaded car. You can buy additives that’ll help, or just get new heads.
I get nostalgic when I watch Adam-12, and see those gas stations with 102 octaine Leaded for 18 cents. sigh
I’m having hardened valve seats put in my 1966 MGB. That should solve the problem.
So it looks like the car’s going to be fine. Darn. I was half hoping for some fireworks.
The last time I looked, some service stations had unleaded fuel and something called Lead Replacement Petrol. Is LRP just a scam to squeeze a few extra cents out of people or does it perform differently to regular unleaded?
Depends on the vehicle- some of the mid-late '70s Chevys had engines that called for leaded fuel, but had hardened valve seats that could handle unleaded with no problem- my old Suburban was one of those.
Most (or possibly all) Japanese cars from the early 1970s will run perfectly on unleaded fuel, because they passed the same laws then that were passed elsewhere during the 80s. Here, it’s the big Australian (and therefore I assume probably American too) cars that have the problem with valve seat wear if built pre-1986.
I wonder if the old valve springs would be strong enough to do the damage - if, of course, the springs were never changed.
Also, add O2 sensors to the list. Beyond the catastrophic converter and the O2 sensor, an unleaded engine should be just fine on a leaded diet.
Less than two tanks of VP C-16 will kill an O2 sensor. The heated O2 may not set a code, but it will not work correctly.
Does the lead plug the catalytic converter or just render it useless? [/related hijack]
<slight hijack>IIRC,you ** can’t ** put leaded (or deisel) into a made for unleaded car,unless you cut the filler neck to make it wider.
At any normal gas station those fill nozzles are different widths with the unleaded being the narrower to accomodate that opening in the filler neck.
So it’s pretty hard to screw up that catalytic converter that way.
Titan, I forgot about that living in the USA. We switched way back in the early 80’s. The only place I know of to get leaded is at the track and we fill jugs; so it goes in with a plastic funnel.
The strange thing is the emissions inspection people still check the filler neck to make sure the car is not modified to accept those bigger leaded fuel nozzles. It is strange because it is not available at the road pumps, and where it is available, it is the race fuel that goes from 4.50 to $7.00/gallon!?!?
The thing they should check is the catalytic converter. That is something that does get altered and in all the years, they have only looked once and I have five cars so it seems that I am always going to emissions.
As I understand it the lead in leaded gasoline fouls the catalyst. The thing eventually burns or booms, can’t remember which.
IIRC the lead will cause the CC to overheat, just like unburnt fuel