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97 Crown Victoria - Knock, knock

Just before the start of last winter I bought a 97 Crown Victoria P71 Unmarked Police Interceptor, 4.9L with 118k mi.
Since I have owned it (and I suspect for some time before that) it has had a very loud engine knock. (The previous owner informed me that he had been running a 50/50 mixture of 40 weight oil and Lucas oil since he has owned it)
The first thing I did was a simple tune-up. Plugs, wires, fluids, filters. No change to the knock. (Plugs were the picture-perfect poster boys for a clean burning combustion chamber)

End of the winter (3 months after purchase), A DTC creeps up.
P0402 - EGR Excessive Flow Detected
Don’t think something like that would cause such a noise, but I take the EGR valve off, clean it and test it, and it seems to be working fine. I replace the gasket and the valve.

I went to check my codes today, and I have 3. Not sure how they’re relevant, but I’ll include for completeness sake.
P0141 - HO2S12 Heater Circuit Fault
P0402 - EGR Excessive Flow Detected
P1469 - Low A/C Cycling Period

I tried to detect the location of the knock with a stethoscope but it seemed to just be loud everywhere. I could not say whether there was a specific side or specific part where the noise was coming from.

At idle, the knock is low pitched and throaty. I’d estimate 240 knocks per minute at aprox. idle of 850 RPM (About 4 knocks per second). The knock increases in frequency and pitch in time with RPM increases. At RPMs over 2k it sounds like someone is under my hood with an air hammer. When the engine reaches operating temperature, I notice the idle knock is a little louder and higher pitched than when cold.

Other than the knock, I’d never know there was a problem. I get plenty of power, the engine idles and runs smoothly, nothing is skipping or shaking.

The only other oddity I notice, and I’m not sure if it’s related or even abnormal, is that my oil pressure gauge always reads somewhere in the bottom quarter of the safe range.

I’ve never had experience with problems like this and would appreciate if anyone could point me in the right direction here.

Based on the noise and the low oil pressure, I would suspect very badly-worn bearings.
If the previous owner was using 40-weight oil, it may have been because he knew about the bearing problem–and of course, this is something that only continues to get worse.

I would suggest that you not drive much further before tearing down the engine for examination.

Did I forget to mention I’ve put 7k on it in the last 6 months :smiley:

That’s the kind of answer I’ve been dreading. Though it does seem likely.

It sounds to me like you bought a car from someone who knew there was a problem with it. You knew there was a knock from the get-go so you should not have purchased this car or not given much for it if you went ahead with the deal.

Odds are it’s an engine that has lower end problems but a rod bearing has a different pitch as compared to a valve lash adjuster, cam chain tensioner, etc.
It could be that this engine has a leaking oil filter adapter gasket and coolant mixing with the engine oil has washed the engine out; meaning it’s toast.
The gasket is 13 bucks but replacing that is the equivalent of closing the barn door after the horses ran off.

These engines are near bullet-proof and if it’s worn out at 118k miles then the engine did not die a natural death; it was a homicide.
I’d start looking for another motor because overhauling one is expensive even for a DIYer and at this point those codes are actually irrelevant.

You have to remember that police cars are left idling for long periods of time, so that mileage does not reflect how many hours are on the engine. It sounds like a worn engine to me, unless there’s a loose or rattling exhaust part. But that’s really dreaming. Maybe you can buy a rebuilt short block and swap it in.

Luckily I got it cheap and budgeted it’s actual blue book value for repairs. Unfortunately, engine noises are not my strong point, so I’m not confident enough to pinpoint this by sound alone, and I’ve heard every crazy opinion from lifters to smog pumps. I did not encounter coolant in the oil and the coolant and oil levels have remained steady as long as I have had it.

Exhaust manifold leak was my very first guess… And I have heard the police car theory.

sounds like could be a front main bearing knocking. Lot of load on the front of crank on a cop car,so ya get a knock with no connecting rods hanging out…

On a hunch (read: stupid friend told me to try this), I unplugged each spark plug one by one, one at a time.
This did not change the noise.

Would it be worth my time to tear down this engine and try to fix it? or would a complete replacement be more time/cost effective?

I second the idea that you have worn bearings. You may be able to pull the oil pan and replace them if you don’t wait until it gets really bad. At this point I’d say you have nothing to lose by trying.

Generally speaking, if a rod bearing is the source of the knock it will go away or subside quite a bit when the spark plug on the knocking cylinder is disabled.

You need to try and determine if this noise is down low or up top. A valve lash adjuster will have a sharper and more tinny sound than a rod bearing would.
A lash adjuster is also not the kiss of death for the engine either. Maybe you’ll get lucky.

Drive it the way it is until you locate a replacement engine. Try to find one from a '97 P-71 so all the wiring and computer will be the same…Or just find another, newer, P-71…In 2005 they got a nice upgrade, rack and pinion steering, new heavier frame, nice 5-link rear suspension, upgraded transmission, outboard mounted rear shocks…

No one has mentioned the DPFE sensor but you may have a bad one from the excessive 402 code. The input from the computer to the sensor is about 4 volts and the output to the EGR should be 1 volt; anyone over 1/4 volt either way will cause a lamp and malfunction. The knock you are hearing may be detonation that will ruin the engine for sure.
Also a sink screen washer (stainless) will reduce carbon buildup in the EGR.

I wouldn’t think an EGR fault would cause a knock at idle on a stationary car.

Let’s see, 850 RPM is about 14 revolutions per second. That means every cylinder is firing 7 times per second. My guess is that it’s knocking 7 times per second at 850 RPM rather than your estimated 4 times per second…every time the cylinder ajoining the spun (or damaged) main bearing fires.

I’m guessing you have a spun bearing or serious bearing damage, perhaps as a result of the previous owner having mixed his own customized oil concoction. Pull the pan, pull the caps, and I think ou’ll find the source of your knock.

I wouldn;t even worry about the codes.


While not to disagree with everyone here, they are probably right, but this engine has an unusual valve train. The cam rides on top of the rocker arm, the valve is at one end and the hydraulic lifters form the pivot point on the other.

There is a ball bearing cam follower in the center of each rocker. If the ball bearing were to fail, it could make a knocking sound. It’s a shot in the dark, but I’d remove the valve covers first and check out the valve train before tearing into the engine. Also check that the rocker hinge isn’t broken.

OK450, sorry I missed the knock in idle somehow. Wasn’t watching close.

I concur w/most everyone here that this seems to be a crankshaft or piston rod bearing problem; i.e. in most cases, fixing this will require the engine to be rebuilt.

There’s a slight chance the cause could be an accessory that is causing the knock, like an alternator or power steering pump or water pump. Unlikely, but for $$$ reasons you should rule this out first.

You said you used a “stethoscope”. But is it one designed for auto engine diagnosis? What I do is take a 3 foot length of thick rubber hose (from an old hose you used for watering your lawn), and hold one end against my ear while I probe the other end in the various places around the engine a knock might be coming from. Most mechanics have something like this they use to isolate the location of noises, so if you aren’t able to do it yourself, ask your mechanic to.