diminishing performance

check fuel pressure and filter should be in carburetor then overhaul carburetor. check radiator when hot with engine off and if electric fans disconnected. check for cold spots if any radiator is bad. also, check radiator cap with a pressure tester.

Would any of that account for the severe drop in performance? Flooring the gas may (sometimes not right away) result in a transmission kick down but even with the secondaries honking (sometimes they do not seem to open right away either), acceleration is slow (like an old diesel engine with no turbo). The car operates at standard operating temperature getting a little hotter going up a long hill but not boiling over into the overflow tank unless turned off immediately after climbing a hill. I can hear the cooling fan running with the engine on.

Anything else come to mind? How about if I just strip it down to the block and heads, get a standard 4bbl and tear out the computer and every other e-device and just call it a day (of course, then it will not be factory stock anymore).

Still perplexed
88

PS: Could the car be operating in the default mode which kicks in just to get you home or to a service station when there is a major problem? The check engine light is not on.

you do not have a default mode on that car it is to old and it is a carburetor. I think if you check fuel pressure 4-7 psi and about a half pint of fuel in thirty seconds. make sue that is up to snuff and then overhaul carburetor as it is forty years old and with the fuel the past twenty years or so would screw that carburetor up as well as how much junk is in the float bowl. It is because of alcohol. the float in carburetor may have been eaten up inside by fuel and accelerator pump is not in good shape as well. the stuff you get today for that is made to use alcohol not the early stuff your hose to fuel pump is probably sucking together as well due to the fuel. that is on both ends tank and pump. that is probably your biggest problem the switch on carburetor does not do a lot so do not worry about replacing with a newer carburetor. you also probably have a choke problem due to old vacuum lines and such they do not last forever.

Thanks HMAC300. I will check that out. You figure that alone could account for these symptoms?
88

Pressure test the exhaust as well. Poor acceleration, running hot can easily be caused by a partly plugged exhaust, especially in an older vehicle.

Thank you, Steve. The muffler is the original factory muffler. It got loud and now has quieted down. The mechanic I mentioned did pressure test the exhaust but made no comment. I told him there was a hole in the muffler (which there has been for a while). The question is, why has it gotten quieter instead of louder. Could the inside have collapsed and partially plugged it up? On the other hand, would a plugged muffler cause all the symptoms I mentioned above? Also, the engine only gets hotter going up a long hill. On a flat course the temp seems to remain normal.

Thanks.
88

Have a back pressure test done most exhaust shops can do this. sounds like you could have a plugged catalytic converter that would explain the bad muffler getting quieter less exhaust going through it.

Louder then quiet with no intervention means something inside the muffler or the converter collapsed. Yes, it would cause the engine to run hotter under load as it uses more gas to create more power so you get more exhaust as well.

Basically your engine is an air pump. On the intake stroke a valve opens and atmospheric pressure rushes into the cylinder. On an engine this air also has fuel in it from either the carburetor or injection system. The piston rises, compresses the charge and the ignition sets it on fire. The piston gets driven down and gives you power. As it comes back up it pushes exhaust out another valve and the cycle repeats as the intake opens again.

With a plugged exhaust the piston tries to force the exhaust out , but it cannot remove it all, now the intake valve opens and you get some exhaust pushing into the intake, that lowers the amount of fuel coming in because it has to force past a small amount of pressure. Plus the exhaust that is still in the chamber dilutes it more. That gives you a loss of engine power and lowers the vacuum signal to things like the transmission and causes it to act differently.
Plus the blockage stopping the exhaust means that hot exhaust gas cannot leave the engine area as fast to help cool the engine. That causes more issues as the cooling system now has to remove much more heat and you get a hot engine.

Easy test: Start the engine, have someone set in the seat, you go back and put your hand over the tailpipe. Have them floor it, you should get a big burst of exhaust that pushes your hand. A blocked system will usually make a hissing sound and produce very little pressure.

Also, check for a restricted air filter on a carburetor that would really effect performance. My friend let his get so bad the engine shut off.

Be careful there can be super heated exhaust coming out the tail pipe.

When catalytic converters get plugged or too much fuel passing through then they can get really hot and in turn produce super heated exhaust gasses.

Thanks to both of you. It would be nice if that did the trick. I was not looking forward to tearing it down chasing those other possibilities and the muffler has to be changed anyway so that is a good place to start. How can I tell if the catalytic converters are plugged for sure? If it is, I will probably go for dual exhausts.

There is a tubular brace that runs from side to side. it is right side is flattened somewhat to allow the exhaust system to snug up a little closer to the floor pan without putting an extra bend in the pipe to get underneath the brace. Can I flatten the other side to accommodate a left pipe as well without loosing strength in the brace? I've heard that cat's have improved since that car was made. Do you know the best kind of cat to use on that car (and why it's the best) and who makes it?

Thanks..... 88

As far as bending to get dual exhaust I could not tell you. an exhaust shop could tell you that. also, a back pressure test has to be done on the exhaust with a back pressure gauge. An exhaust shop could do that for you.

Hello 88,

Here is a link to an excellent article from this site with step by step instructions, pictures, and a video on how to test a catalytic converter.

https://www.2carpros.com/articles/how-to-test-a-catalytic-converter

Thank you for visiting 2CarPros.

Regards,

Renee
Admin

Thank you all for the tips and the info. I'm going to disconnect the muffler at the cat and have the bolts soaking in penetrating oil in the meantime. If there is no difference in performance, I'll disconnect the cat from the front exhaust pipe and see if there is any difference. Can you tell from the sound of banging on either the muffler or the cat if it is clogged or collapsed inside?

thanks,
88

Usually not unless the core of it has broken loose and chunks of it moved and caused a clog.

Okay, I have removed the muffler and pipe up to the catalytic converter and there is no change in performance or noise. It is still quiet. I am taking off the catalytic converter next and have the bolts/nuts on the connections, clamps, and hangers soaking in penetrating oil. There is a stainless steel tube running to the catalytic converter from above which I am guessing is the air inlet. I have removed the small clamp which appears to be holding the tube to another tube coming out from the side of the catalytic converter. It appears to tighten onto a short connecting tube which slips over both tubes at the junction. This small connector tube will not move either way. Does the catalytic converter have to be disconnected at both ends to slip it free from this connection (could be difficult holding up the cat while completely disconnected and working the tubes apart without doing damage to the tubes or tearing the small tube's connector into the catalytic converter)? If there is a sequence to removing this catalytic converter please let me know. So far I have not been able to find such a sequence on the net for this cat which is the original factory part (1988). This one has no O2 sensor. Also, I have looked into the exhaust end of the catalytic converter and it looks fine (I have seen photos on YouTube of collapsed catalytic converters similar to mine and of good ones). There could be damage at the other end or it could be clogged with carbon at the front end. I will not know until it is off. I will also be able to test drive it with the catalytic converter off to see if the power comes back. If it does not, I do not know what to suspect next.

88

Why do you not just unhook the down pipes off of the exhaust manifolds and see if that makes a difference.

An elegantly simple solution I have overlooked. Thank you. I can unbolt the front side of the catalytic converter which is easier. If I can move it clear of the exhaust I can test it like that. I will try it first thing in the morning.

88

Cool, it is easier for you to see what is easier since we do not have pictures or the ability to see what your seeing .Keep us posted.

Well, this catalytic converter is being very difficult. The flange connection at the exhaust end of the catalytic converter serves to connect the pipe and muffler (which is off the car) to the catalytic converter and the catalytic converter to a rather substantial hanger complete with cushioned mounts. One of the two bolts holding the catalytic converter to the hanger finally came loose using an air gun and socket, the other has not budged after repeated tries and soaking with penetrating oil. The front appears to be held in with an exhaust clamp which was placed with the nuts on top making it very difficult to break them loose or even to saturate with penetrating oil. Space is very limited and I cannot get anything long in there to help me apply torque to the socket and ratchet. I have tried tapping it but space is too limited to get a good swing with even a small hammer. I am thinking of just cutting it off if I can get my tool in close enough with an abrasive wheel without damaging everything in its vicinity. There is stainless steel tubing to both the cat and the transmission very nearby. Why a flange with adequate room for access was not used on that end is beyond any reasonable logic.

There is a lot of soot in the pipe on the exhaust side of the catalytic converter. The interior looks good but I noticed almost exactly half (right down the middle) of the material inside appears be sootier than the other half. When I rev the engine I see smoke billowing out the back; something the car has never done up to now.

And so it goes.

88

You can buy a exhaust back pressure gauge that you can measure the actual exhaust back pressure.

Hi guys!

First, thanks very much for your enlightened suggestions they have led me to solving the problem. I am glad I took the time to keep investigating and talking to you guys before jumping in. It saved me a lot of money and labor.

It turned out to be a plugged catalytic converter, as you suggested. The car runs much better after testing it without the exhaust and blowing out the built up carbon. All the previous problems we discussed, the high speed idle, the dieseling, lack of performance are all gone. Now I need to replace the muffler and the catalytic converter.

Just for information for anyone else reading this string, the three way catalytic converter I have on my V8 has three chambers separated by screen. The first chamber (entry chamber) was crumbled. After getting it off I shook it and heard the pieces and dust rattling around and poured them out. Unfortunately, I was unable to detect this while still on the car (like by tapping). Taking it off was a chore as one of the rear hanger bolts was quite seized, but perseverance with an impact wrench (and lots of heavy duty penetrating oil, tapping, and a little heating and any other trick you know) finally, after several hours, did the trick. The intake side was held on with an ordinary u-clamp which I finally had to cut off (I got sick of playing with it by that time and decided my time was worth more than the price of a new u-clamp). I also had to cut length ways along the short stainless sleeve on the air tube to free up the air injection tube on the side of the catalytic converter. Go slow and take it easy so as not to cut into the stainless tube (I cut mine a little but it can be easily fixed by using a slightly longer sleeve) that carries the air from the air pump (if you have one) and peel it away along the cut so the tube on the cat slips easily out as you pull the cat loose from the pipe coming from the manifolds (the intake side) which is not easy on an old car (mine has 120,00 miles on the original equipment). But, again. a little tapping (do not dent it or it will be harder to slip off), penetrating oil and shifting back and forth and up and down finally got it loose.

So, that is my story and I am sticking to it! Any knowledge about where to get a 23 1/2" end to end (including exhaust end flange) three way catalytic converter as good as the original will be wholeheartedly appreciated (I know good luck, right?). Apparently, GM does not supply them anymore.

One last thing. I read the spec's on the Aero Turbine muffler 2525XL and wondered if it is a good choice. It is all stainless steel and comes with a lifetime warranty. Supposedly, it has no baffles for low back pressure and uses wave cancellation to quiet the exhaust pulses. Aeros are primarily for diesels, by what I have read, but this one looks like it should work fine on my V8 unless it is too loud (it is a luxury model so I do not want it sounding like a hot rod). The XL is supposed to be a quieter version of the 2525.

Thanks,
88

Glad to hear it was a plugged catalytic converter like I suggested. i thought it was. Check out rock auto on the catalytic converter. i have a five percent discount code to them if you are interested. I buy parts from them all the time. As far as the muffler goes i do not know anything about those mufflers. I had a Eddle Brock Aero chamber performer muffler on my s10 Chevrolet great sound. Flow master is nice also.

Great to hear it. Those parts sort of fuse together over time, I do not even try to save most of them, just grab the plasma and slice them off.

Most of the newer converters they sell to fit that car will do the job.

Muffler wise, I have been using the Walker stainless ones on my vehicles. Quiet, flow well and seem to last okay. The issue with stainless is that it will rust, just slower that plain steel. So I get the walker ones, lifetime warranty on them and I use band clamps instead of U clamps so they can be removed easier when the pipes rot off.

Thanks, Saturn. I checked out the site. It is one I have seen before in my search for parts. They seem to have a good selection. And thank you, Steve. I have checked out that Walker with the lifetime warranty and it is certainly a contender.

Just a note on my last comment. The sections are not separated by screen. It looks like screen in the front and back with the limited visual peering through the hole but it is the honeycomb upon further inspection.

Thanks again.
88

I have been buying parts from rock auto for years I love that place.here is my five percent discount code when your at your shopping cart put it in the how did you hear about us box. Here is the code and thank you for using 2carpros and please tell all your friends about us.



This is the discount code bellow.
6529518453612978

That is very generous of you, Saturn. My thanks, once again.

88

You are welcome and thank you for using 2carpros.

Just another quick question if I might. This car is now running almost perfectly now that the exhaust restriction is gone. However, when I put my foot into it I cannot always hear the secondaries honking. If it kicks down into first gear I think I can hear them. I understand they are governed by the computer which decides if the engine needs more air for proper combustion (Rochester quadrajet). Also, there is frequently a slight stutter sometimes when taking off from a complete stop.

Thanks,
88

As far as the stutter for taking off from a stop I would see if your accelerator pump is squirting gas when opening the throttle?

That makes sense, but I can see/hear/smell gas squirt when I activate the throttle with the engine off.

88

Any thoughts on the secondaries?
88.

Those carburetors were junk when they were new. you could very well have a bad carburetor. That would take out a catalytic converter and cause performance issues

If your timing and ignition system is good I would be looking at the carburetor.

Thank you, Saturn. Yes I have read about them and there are mixed feelings about the design. Some dislike them, others say these later ones are okay. Mine is the original, untouched, so it may very well be ready for a rebuild or at least new gaskets, but overall it runs pretty good and gas mileage, now that the exhaust is not clogged, also seems to have noticeably improved. Soon as the gas prices come down again, I will fill it up and check the mileage more accurately.

If it is any help, I have noticed (because I am listening more closely now) that the secondaries may be opening mid way through a full throttle acceleration, if that is any clue. Maybe it is designed that way since fuel economy was a driving force in those years. Anyway, it is no big deal if the gas mileage is good. I am not racing it and it has plenty of pick up for passing. I just thought you might know a couple of easy tweaks for best performance.

thanks again and Best wishes,
88.