Best of Deals Car Reviews Repair Shops Cars A-Z Radio Show

Detonation in 1991 lincoln town car

My car is a 1991 lincoln town car with 58000 miles. I have been having a problem

with knocking and pinging. This happens most right after the car shifts about 25-30

mph. My mechanic searched all data and saw a bulletin for this suggesting maybe

the cpm needed to be replaced. Another mechanic I know said maybe the problem is

carbon build up. I want to fix it right, but am not sure what direction to go in.

I went to the dealer also they said basically the car is to old and they can’t

do any upgrade or replacement of the PCM. Any guidance that you can offer would

be greatley appreciated!

What is the displacement of the engine in your car?

Mileage is low for a carbon build -up condition. Have a expert ride with you and try to determine level of knock/ping.

Has the noise always been there? Like it is a result of a engine build mistake?

Are you using the octane of gas required? Have you tried filling at several different stations? What about the vehicles prior service history,are the plugs and wires correct? Is it possible to change the ignition timing? vacuum advances hooked up properly? Does the car run-on after you shut-off the key?

What engine are you using? I would like to do some reasearch

All emission related equipment installed and functioning

Do a compression test when you check-out the plugs

Post any bulletins you are working off.

It’s a 4.6L

Those call for 87 octane, but I’ve found that they really need 89 octane.

It is, indeed a “regular gas” vehicle. Putting in some higher octane fuel might help diagnose the problem. Try it and see what happens. There might be an “octane pin” on the PCM that pulled out or jumped to stop the pinging. I have a vague recollection about it. You should try other ways to fix it first because that just retards the timing which will result in lower power and reduced mileage.

Does this happen when the engine is relatively cold? What does the temp gauge do, is it in the normal range (middle)? You may need a thermostat if the temp is a little high that might cause pinging. is a great resource for you to maintain this vehicle. If you don’t get anywhere soon, try posting over there.

On a non-distributor equipped car such as yours the usual cause is an EGR system fault.
There are procedures for checking the few parts in the EGR system and it’s not that involved. It’s also quite possible that since you have an older car like this with super low miles the EGR system passages have become heavily sooted up over the years due to limited use of the car. Think of a chimney that soots up and needs to be cleaned.

I think they’re barking up the wrong tree, and probably an expensive one, by blaming this on the PCM.
One would think that one of these people you’ve dealt with would have thought of the EGR system.

The EGR system used to cause this when it failed to open.

These early 4.6L modular engines used a small tube to supply exhaust gas to the EGR valve. It plugs up easily. I have a '92 Vic with this engine and my CEL is on perpetually because of “Low EGR flow”. I don’t suffer from pinging or detonation however…Perhaps the “anti-knock sensor” has failed or become disconnected which allows the ECM to advance the timing too much…

You might try a can of BG 44K added to a 1/4 tank of fuel, then take the car out on an open stretch of road and “blow the cobs out of it” …

I sure miss those days of clean intake manifolds with no pile of widgets on top of it and 2 vacuum hoses.
That Mark of mine has the DOHC 4.6 and I don’t think a glass of water would run out the bottom if it were dumped in the top of the engine compartment.

Here is the EGR valve. It has a place for a vacuum hose. Check the condition of the vacuum hose.,239443/vehicleId,1610301/initialAction,partProductDetail/store,1140/partType,00214/shopping/partProductDetail.htm

I don’t think that the early 4.6L’s had knock sensors.