Popping noise

[Dan Cordon]

Doug wrote:

> Recently I've been noticing a loud popping coming from the engine
> compartment of my '91 100.  It happens when there is an increase in load
> on the engine:  when going up large hills, accelerating in low gear
> (usually from a stop when I try to accelerate hard - have to take it
> easy), or worse, accelerating up a hill.  Because it seems to be related
> to load, I'm thinking it's a leaking vacuum problem (I had the manifold
> apart recently to change the valve cover gasket, may not have clamped
> something down enough?)  I'm going to check some of the hoses this
> afternoon, and was wondering if anyone had any other ideas.
>

You may be on the right track. Popping from the engine is usually a lean
misfire. The cases you mentioned could all be before the turbo produces
boost. If that's the case, and there is a vacuum leak in the manifold
then the engine could be digesting more air than the airflow meter is
measuring.

Oh wait....your 100 is probably not a turbo. So, that could definitely
be the case. If you have air leaking in after the airflow meter, you'll
likely get popping from the engine. I would actually look at hoses
before the throttle body. If you had a leak after the throttle body, it
would run lean all the time (as there's high vacuum in the manifold).
Where there's really not much vacuum in the rest of the system unless
the throttle is open pretty far. I don't know the plumbing well on the
NA cars, so I can't recommend exactly what to check.

What you can do is use something like starting fluid and spray it at
potential leaking components. If there is a leak, the engine will
increase speed after spraying. If you don't like starting fluid, you can
also use something like propane too.

Good luck-
--
Dan Cordon
Mechanical Engineer - Engine Research Facility
University of Idaho