89 240DL Wagon, M47, Knock-preignition problem, UPDATE

ToeCutter66 · Post by **ToeCutter66** » 14 Nov 2006, 17:14

Hello all,
I have an 89 240DL Wagon, M47, 135k miles. If I am on the highway in 5th gear with light throttle the motor pings/knocks (around 2500 rpm). If I take my foot all the way off the gas and then press the throttle 1/2 - 3/4 the way, it stops instantly, and will stay away for a while. There are no codes in either the fuel or ignition system. This car mostly sees city driving, I use 87 octane gas and it otherwise runs very well. I am about to drop the $50 on a new knock sensor, does anyone have any other suggestions?

Thanks
Kevin

Kmaniac in California USA · Post by **Kmaniac in California USA** » 14 Nov 2006, 18:56

Start with the basics. The knocking is a symptom of a lean air/fuel mixture. A lean mixture can be caused by a vacuum leak. Check the integrity of your vacuum lines and replace any that look cracked or ar leaking. Also check the integrity of your vacuum reservoir tank, as well. Next, check your intake manifold to head connection to verify the integrity of the intake manifold gasket and retorque or replace as needed. Do all this before replacing any parts, including the knock sensor.

ToeCutter66 · Post by **ToeCutter66** » 15 Nov 2006, 07:02

Chris,
Thanks for your input. I will check tonight for vac leaks. My spark plugs are on the whitish side, so this seems to confirm your diagnosis of a lean running condition.

ToeCutter66 · Post by **ToeCutter66** » 15 Nov 2006, 15:48

I checked for vac leak. I tried with both brakecleaner and propane. I tested it at the intake manifold, injector seals, throttle body, bellows to AMM, brake booster etc. There was no change in idle. Any ideas on what to check next?