I. this study was commissioned between 1993 and 2005. the factory service manual does an excellent job of explaining how to test and replace components, but does a poor job of explaining any symptoms of a failed part.



II. the cars. all the cars tested had engines with normal factory spec compression. the compression number isnt important to this test as long as its even. the worse the engines health, the more a given component failure will affect it. these tests were also done on stock or nearly stock cars. the survey cars vary in years from 1979 models to 1995 models.



III. the testing;



A. Fuel system:



1. fuel tank; vary rare failures, aside from external leaks, they can fail in 2 ways. 1st one is rust from sitting, which will clog the filters/pumps etc, second one, in very very rare cases is the fuel tank baffles breaking due to extensive hard cornering. the broken baffles will rattle, and cause hesitations due to fuel slosh, but its very rare.



2. fuel pump sock; this is the filter that is on the fuel pump.

since it sits in the tank, its got a dirty job, clogging of this filter will cause a lack of fuel flow, and a lack of high rpm power



3. fuel pump. pumps fuel. if this doesnt work, car no run. fuel pumps also can get weak over time, resulting in a lack of fuel volume and high rpm power



4. fuel lines. the mazda hose is excellent, and should outlast the car. if the hose has been changed for some reason, it will have been replaced by worse material needing frequent inspection



5. fuel injectors; thses fail in several different ways. external leakage, its possible for the injector to leak fuel externally, the FD in particular leak thru the electrical pins. internal leakage. the injector can leak fuel when its supposed to be closed, if the engine is running this is not generally a problem, how ever it causes flooding and hard starts when the engine is turned off.



6. pulsation damper; these just pretty much leak externally, on a non fd its kinda rare failure, on an fd its a certain failure. big fire hazzard



7. fuel pressure regulator: regulates the fuel pressure. it is a rubber diaphram with high pressure fuel on one side and either vacuum or boost on the other. the failure is a break in the rubber. this causes a, incorrect fuel pressure, and b, fuel can leak thru the diaphram and into the intake manifold. when these fail the car generally runs really really badly if at all.

B. air intake system, this includes everything from the intake ducting to the manifolds to the engine.



1. air filter; this has a replacement interval of 30,000miles, if its clogged up the car will loose power, and mileage. if you're using an aftermarket filter, care should be taken to use one that has more filter AREA than the stock filter.



2. air flow meter. this is the device which the ecu uses to calculate the air flow of the engine, and thus the amount of fuel it needs to supply. the 89-91 cars had a bad batch of afms, and the updated part can be id'ed by a white dot. we had no recorded failures of a mazda air flow meter, however its very common on bmw's. symptoms vary, but primarily are hesitations and stumbles.



3. air intake tube; this is the tube or duct from the air flow meter to the turbo or throttle body. on an air flow meter car, this tube or duct is hugely important, the air is measured at one end and consumed at the other. if the amount of air measured is wrong, then the engine will run poorly. usually leaks are noticable near idle, and usually a low or non existant idle is the result.



4. throttle body; there are lots of component parts to these, but they will be taken seperately. throttle body's are actually very good, most of the failures were to due carbon buildup, resulting in sticking throttle blades and an intermittant high idle.



5. intake manifolds; the only failures really possible here are stuck 6 port valves and sticky vdi valve. usually this is from carbon build up. if the valves stick closed, there will be a lack of high rpm power. if they are stuck open, idle might be a little low and lumpy, also maybe hard starting.

Problems most of us know about. LOL

How does carbon build up in the throttle body, if fuel doesn't pass through here?

not exactly carbon, more like a sludgey mix of oil and whatnot. A big reason, on rotaries, for buildup in the intact is the overlap of the exhaust/intake "stroke" some exhaust is pushed into the intake. Plus on a bonestock car you have the EGR's sending exhaust gas back into the intake manifold, PCV's venting the crankcase into the intake etc, tons of crap is pushed into the intake to reburn and thus less pollution.



kevin.