Cold-Start Carbon-Knock

Partial workaround

Design fault only on earliest KL V6 engines with engine #s pre KL326296, pre 08/03/93. Mazda TSB B00995/95, issued 16/06/95, re 626/MX6/Probe-2.5.

Symptoms

Diesel-like sound on winter cold-start, diminishing quickly as the engine warms (sounds like frozen peas bouncing off pistons).

Problem

Carbon-build up causes "carbon-knock", tackle when heard. The noise is not HLA/Lifter ticking, nor the (aesthetic) Friction Gear noise localised around the exhaust cam of the rear V6 bank, timing belt end and dissappears at circa 2200rpm.

Solution

Simple decarbon service at £70/70$US, common for many engines with modern fuels.

TSB solution, #96113, May 96

1. Bring engine to normal operating temperature, then shut engine off.
2. Use a 610mm (?!) length of 1/8" I.D. vacuum hose as a feed tube for carb cleaner (Carburetor Tune-up Cleaner, ford # D9AZ-19579-BA), although it is strongly advised to use an aerosol cleaner to eliminate the risk of liquid lock (some techs have done it). Do not use any of the vehicle's vacuum hoses for feeding cleaner into the engine otherwise they may deteriorate as a result (vacuum leaks on 24 possible hoses is tedious).
3. Connect one end of the feed tube to the Purge Control vacuum port on the upper intake manifold and place the other end of the feed tube in the can of carb cleaner, or to the spray nozzle of an aerosol. If a liquid bottle is used the feed rate must be at least 30 secs to avoid hydraulic lock-up damage.
4. Pinch the feed-tube with locking forceps or locking pliars to prevent suction during start-up.
5. Restart engine.
6. Allow idle to stabilise, then feed the cleaner into the engine by releasing the locking pliars/forceps while lifting engine speed to 3000rpm until half the cleaner is used. If using liquid feed, ensure feed-rate is always erring on the safe side by kinking the tube (a thumb screw clamp is ideal). If using a spray feed (preferred) keep shaking the can vigorously. At the half point allow the engine to stall, or shut-it off.
7. The vehicle should be left to sit for 6-24hrs allowing the cleaner to saturate the carbon, the initial intake of cleaner loosens carbon by both dissolving and also by a cold-shock.
8. Flush remaining carbon by repeating the above steps using the remaining half of the carb cleaner. Do not let the engine stall during the procedure.
9. Remove the feed tube & reconnect the purge control vacuum line.
10. Change oil & oil filter to remove contaminants & carbon in the oil which will otherwise be pumped through all the HLAs.
11. Take the car on a 3-4 mile test drive, using the full-rpm range to remove any remaining cleaner or carbon from the engine.
12. Let the car sit for another 12hrs, restart - if the noise is gone then the cause of the noise was carbon build-up.

Note the above procedure is a little complex and requires an oil & oil-filter change.

Full Noise TSB re Carbon-Knock/HLA/Friction-Gear

There is a full TSB re Noise from Carbon-Knock/HLA/Friction Gear:
p1 p2 p3 p4 p5 p6 p7 p8 p9 p10

Can I use higher-octane fuels for more performance?

Using high-octane fuels (98/100 RON) has benefits on the V6 engines as they have a knock-sensor to take advantage of it; UK Standard unleaded is 95RON, Super-Unleaded is 97-98RON. However using higher than 95 RON is largely unnecessary (re cost/power).

The ECU automatically adjusts timing based on the knock sensor from 6-18 degrees, a very considerable range. Base timing is 10 degrees +/- 1 degree.

Cars with 2.0 engines, like Miata/MX5s, do not have knock sensors and thus can't take advantage of higher-octane fuels without adjusting their timing. The miata/MX5 groups often do this, but there is no safety back-track if pinging does occur due to carbon-buildup (on any engine) or simply poor quality gasoline a risk of engine damage as a result.

TSBs?

The specific Carbon-Knock TSB refers to 'customer satisfaction'; some mechanics say if untreated long-term then valves could be damaged (burnt). Decarboning is an easy & relatively cheap service to performand benefits any engine, not just those early V6s whose head design allows carbon to build-up in an area where it will create noise.

Costs:

Warranty, Cleaners in tank/carb-cleaner or Decarbon Service.

Last Upload: 10th July, 2002. V1.50a
URL: http://homepage.ntlworld.com/dorothy.bradbury/probemx/p_p6.htm