'97 Mercury Grand Marquis runs on regular after repairs but but not for long

My car was pinging unless hight octane was used. Merc dealer said they put additive overnight. It ran great on regular after that, but it now pings again unless I go to higher octane. I think they put something to clean out the cylinders. What can I do to do the same? Cost me over $200 at the shop.

What did the manufacturer recommend for that car/engine? If they recommended high test there is little you are going to be able to do other than use high test.

What kind of additive did the garage use? Was it intended to clean the combustion chamber to lower the compression and reduce the octane needs of the engine or was it a octane booster to increase to octane rating of your fuel?

The manual says it’s supposed to run on regular, and it did.

Then it started pinging more and more so I started using the next higher grade. I tried many brands of regular, but to no avail.

The dealer wouldn’t say what they used, but whatever it was, they had to leave it sit overnight. I guess it cleaned the carbon in the cylinders. It ran great after that, but now the pinging has started again so I’m back to higher octane.

Any advice would be appreciated. Can’t keep spending big bucks for some sort of flush.


My guess is the cylinders have built up carbon deposits, so the cylinder compression has gone up, which would be why you need higher octane in order to eliminate pinging. I might suggest using SeaFoam on your engine, which will clean out all the crap that’s built up. For instructions, go to http://www.crownvic.net/ubbthreads/ubbthreads.php?ubb=showflat&Number=1222785&page=0&fpart=1

Thank you for the recommendation. I’ll give it a try. Where are the good old days when you could look under the hood and actually see pavement when you looked down into the compartment.

What weight oil are you using? The reason I ask this question is that I had a similar problem with my 1978 Olds Cutlass with the 260 cubic inch V-8. I had the problem with the engine pinging like a marble tournament. I would pour some Casite Motor Tune-up through the carburetor with the engine running and the problem would clear up for a while, but return again. Finally, on a Car-Talk, some caller had a similar problem. Click and Clack asked about the weight of oil being used. Apparently, some G-M cars of that vintage developed that problem with certain brands of 10W-40 motor oil. Apparently, the polymers that were in this oil to give a multi-visosity range caused carbon build-up on the pistons. GM apparently revised its recommendation and said not to use 10W-40. I had been using 10W-40, but after hearing this program, I switched to 10W-30 and haven’t had a problem since. That was 20 years ago and I still have the car. I’m certain that your car is fuel-injected, so you shouldn’t be getting the carbon build up that one gets with a carbureted engine. However, be certain that you are following the manufacturer’s recommendation for the oil.

Already having this problem with 3 different 4.6 engines I’ve always found the problem is in the egr system. Not sure about the 97 but probably has carbon in the egr valve. If so clean or replace it and install a stainless kitchen faucet strainer washer in the intake to the manifold. You may have a DPFE valve on that model that will give too much or too little exh return to engine. Good luck, LEE

Dealerships generally use BG 44K as a deposit remover. Available at NAPA and CarQuest parts stores or online. An EGR fault will make the “check engine” light come on. A hard full throttle run for as long as you can manage it might help clean things out…

Your car has an electronically controlled ignition advance system that MIGHT be malfunctioning. A sharp mechanic can check to be sure it is operating properly…

Sounds like an EGR system fault to me.

4.6 L up until '98 were susceptible to clogging of the EGR system. In 98, changes were made that helped. I would think that you would get a CEL if there were an EGR problem.

Your carbon problem could be coming from the valve stem seals. Early 4.6 L did not have the best valve stem seals, but I believe that was fixed prior to '97. (Ask at the link that lprocter gave you. This a great place for all kinds of info about your car.) How many miles on your land-yacht? How long between the dealer-cleaning and the return of your symptoms?

Note, you should be using 5-30 or 5-20 oil.

Some of the Grand Marquis’ PCMs have an “octane pin” that can be pulled. I don’t recall the model years. Others can have the advance reduced by reprogramming the PCM. This will reduce timing advance. This will also reduce mileage and power so before anyone does this check out all other possibilities first including EGR, cooling (thermostat, radiator, fans), crank position sensor,… The instructions for he reprogramming indicate that it is irreversible.