Time To Replace The Cat?

[albert champion]

had the damndest thing happen twice today...

just finished completing major service of 1987 560sec at 80,000 miles. that included replacement of fuel filter, air filter, oil and filter, spark plugs, spark plug wires, distributor cap, rotor, plus other fluids and rear brake pads, engine mounts, tranny mount, flex disc and tranny filter.

in normal driving, the car seems to have appreciated this renovation.

but, in an attempt to pass on a two lane road i encountered this strange behavior:

at 50 miles an hour, moved out to accelerate and pass. smashed accelerator pedal to floor. kick down kicked in. but at about 65 the engine seemed to have hit a wall. the engine wasn't misfiring, but it would not accelerate to a fully engaged throttle. backing off on the throttle and then using partial throttle, the engine resumed accelerating, albeit at a slower rate, and the pass was completed.

now, for the continuation of that mid-day trip[about 30 miles] the engine and tranny seemed to have no inhibitions at running at speed and accelerating in traffic. although in these instances, there was no pedal to the metal acceleration requirement.

mystified, i took the car to a quiet and unpatrolled bit of straight road. from 0, i began a pedal to the metal acceleration run. everything was fine until the car reached 60-65, and then again it ran into this wall where it would accelerate no further and behaved as if all cylinders were not working. but once again, letting up on the throttle allowed the engine/tranny to begin operating normally and under less than full accelerator engagement the engine was able to take the car to as fast as i wanted it to go.

no other engine operating anomalies that i can notice.

so, i am thinking that the cat elements are collapsing as i once had a somewhat similar experience with my 1986 560sel at 220,000 miles. however, in that episode, there was no way to get the engine to move the car faster than 40mph.

and i have been considering the fuel pump, as this sec is still running the original fuel pump. but my experience with a failing fuel pump in the sel was that when it failed it really failed[i.e., no fuel to the engine].

so, i am all ears if anyone has other explanations for what i have just encountered. thnx.

[haasman]

albert

So it is speed related more than RPM related? Hmmm.

Haasman

[manny]

albert

There are two quick checks you can do to verify your suspicions:
1. Hook up a vacuumgauge temporarily, floor the pedal & observe
You should see 0 or close to 0 vacuum ( if the engine can't exhale, it can't inhale either).
2. Now it get's a little trickier. You can get a back-pressure reading ahead of the cat, if you have the appropriate gauge. Not sure what the spec's are on that engine for back-pressure, but maybe somebody else does.
Good luck.

[stevebfl]

The easiest test for a plugged cat is with a vacuum gauge. Measure the vacuum at idle and then slowly increase engine speed to 2000rpms. The vacuum should be greater at 2000 unloaded rpms. If it drops then suspect a cat.

My first impression is a secondary ignition deficiency. Either running out of available voltage or finding a shorter path to ground as seconadry requirements go up. WAs that a Bosch rotor? If not or even if it was try the old one again.

My cat's a Calico and I wouldn't replace her for all the tea in China.

Use the vacuum gauge, it's quick and a very good indication.

Good luck,

[albert champion]

boy am i an idiot.

haasman steered me in the right direction. i wasn't paying attention to the tach. 60-65mph in 2nd is 6 grand.

somehow my upshift points had been altered - i like them to shift at 5800-5900 - so in kick-down 2nd gear the tranny was letting me run to the redline. and then the rev limiter would engage. had never experienced the rev limiter functioning before.

glad it did, otherwise i would probably have scattered lots of expensive engine parts along the highway.

so, readjusted the tranny. now it shifts just where i think it should, almost at the redline, but not beyond the redline.

thnx for all your suggestions.

[albert champion]

boy am i an idiot.

haasman steered me in the right direction. i wasn't paying attention to the tach. 60-65mph in 2nd is 6 grand.

somehow my upshift points had been altered - i like them to shift at 5800-5900 - so in kick-down 2nd gear the tranny was letting me run to the redline. and then the rev limiter would engage. had never experienced the rev limiter functioning before.

glad it did, otherwise i would probably have scattered lots of expensive engine parts along the highway.

so, readjusted the tranny. now it shifts just where i think it should, almost at the redline, but not beyond the redline.

thnx for all your suggestions.

[haasman]

Albert

Glad to hear it is fixed!

If I may ask, what did you do to correct the problem? You mentioned adjusting the transmission-

Haasman