Wade T

Maybe you guys can figure this one out. The car runs, pulls and sounds good below 2k RPMs but above that the exhaust note gets muffled and the power comes on in a inconsistant manor under normal driving. At the WOT it seems to pull fine and has no issues other than being slow. But, all low throttle action above 2k requires small amounts and varying foot feed to maintain a set speed. Traveling up hills it's more noticible. The car is an 82 5 speed with a Borla cat-back. It's also had a recent fuel pump, filter, and igniton upgrades.
I was thinking the problem was a clogged cat. Any ideas?

Thanks.

atb

Sounds about right Wade. I'm pretty sure we could get one of the other local 16v owners to come down to the shop and do a quick exhaust swap to see if that has any affect on how your car drives. Of if you want to just drop the exhaust we could do that and you could take a lap or two around the industrial park where our shop is open header style.

AO

Did this happen all of a sudden or gradually? What do you mean by "ignition upgrades"?? If you changed plugs, cap, rotor, or wires, I'd double check all of those items first to make sure you didn't get bad parts. Also, when was the last time you checked your timing? If you did it recently, did you disconnect the vacuum? Did you remember to reconnect it? Also, I'd check for any intake or vacuum leaks.

Wade T

Andrew, the issues came with the car. Back then it also had a cylinder miss from a corroded spark plug. The other ignition upgrades were OE replacements except the wires. They were all in bad shape. Haven't checked the timing yet. I'll check it this weekend.

Adam, PM sent.

dr bob

There's a vacuum gauge test that's pretty good at identifying plugged cats. You'll want toi plug the gauge into real; manifold vacuum, so the easiest way is usually to tee into one of the sense hoses to a fuel pressure regulator or damper. Bring the hose for the gauge into the car so yopu can see it safely while driving. Normal vacuum readings show high vacuum at idle, and also when you are driving with a trailing throttle (letting the engine slow the car down). Conversely, you'll see lower vacuum that's related to load and RPM; High load and lower RPM causes vacuum to almost disappear. Moving the pedal will cause the gauge needle to move in direct response. With a good cat and free-flowing exhaust, pressing the pedal to one stable position will cause the car to accelerate and the vacuum will drop. Pedal still stable, as the car comes up to the available speed, the acceleration drops off and the vacuum gets higher. This is the difference-- with a partially plugged exhaust, the car will come to the available speed, no longer accelerate, bu th evacuum will remain low or non-existent.

Remember that the engine is a big pump, and the ability to push spent gases out means that there's room for air to be drawn in. A partially-plugged exhaust shows a much steeper increase in exhaust pressure as you try to pump more exhaust mass through the pipes with more RPMs and engine load. You'll reach a point where a significant part of the exhaust charge won't go out of the chamber, and therefore there will be less vacuum when the piston goes down during the intake stroke. Less vacuum means less air (and fuel) are drawn into the cylinder, and therefore less power is available. Watching the available vacuum is a telltale that the exhaust is partially plugged.

Be aware that a few other things will generate a similar vacuum gauge response. Principal is changes in cam and valve timing, such that cylinder filling and exhausting is inhibited because the valves aren't open at the correct times. To a lesser extent, ignition timing and to an even lesser extent fuel mixture will affect your readings.

Verify your cam timing before you get real serious about exhaust problems, if only because it should be part of a good every 15000 mile timing belt tension PM program anyway. Tension is checked with the engine at #1 TDC, so you can pretty easily see that the marks on the gears line up with marks on the housings behind the gears when setting/verifying belt tension. A good ignition tune-up may also be due. Plugs are easy and inexpensive, but a full service needs to include caps rotors wires if they are over 10 years or 100k miles old. You replaced some of that stuff recently, there's more though.


If the vacuum gauge check doesn't show a partially plugged exhaust, replace the fuel filter, and verify fuel pressure and fuel pump operation. Some cars have two fuel pumps, and failure of one will sometimes give your symptoms. The in-tank pump is often the guilty part(y), mostly since it's the tougher one to change. The possble-causes list probably goes to mass airflow sensors next, temp sensors, etc related to fuel injection. There are even more things to get into, and they are too expensive to just throw parts at them and hope you land on them.

Mrmerlin

If you find that either one of the fuel pumps is bad you should replace both of them, usually one kills the other