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Diminishing performance

Tiny
88
  • MEMBER
  • 1988 CHEVROLET CAPRICE
  • 5.0L
  • V8
  • RWD
  • AUTOMATIC
  • 120,000 MILES
We have had previous discussions about this car. Though the car performs in a diminished capacity but adequately on level ground, it is getting more and more difficult to climb long hills, after which the engine heats up hotter than usual and can sometimes boil over if turned off immediately afterwards. It cools down if left to idle for a minute or two before shutting down after a long climb. At slow speeds on level ground it surges and slows, the idle does the same thing at a stop with the car in drive and feels like it may stall on the ebb of the cycle but idles up and even when taken out of gear, although the idle is not as smooth as it used to be and is higher than spec's. Refer to past posts for more back ground and measures already taken.

After some research and discussion, I thought it might be the timing chain but that was ruled out along with a head gasket by a mechanic who test drove it and did some basic checks in the shop. Then I thought it might be the throttle position sensor (which it does have) and he ruled that out as well. He told me there is little information on that model and he didn't have an electronic module that could do a read out on that one anymore. He gave no suggestions as to what it might be.

My other vehicle is almost ready for the road and I planned to work on this Chevrolet once the insurance is transferred. But now I do not know what to work on. If it is not timing or TPS, if it is not a head gasket, in view of what has already been done what on earth could be causing these symptoms? The engine is good. It's quiet and doesn't burn oil even on start up. I have noticed, with the right side windows open that it will ping a bit going up hill (if that is any help) and stop and ping a bit and stop. It is an H series engine equipped with a quadrajet electronic carburetor.

Perplexed.
thanks, 88
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Monday, June 12th, 2017 AT 11:11 PM

36 Replies

Tiny
HMAC300
  • MECHANIC
  • 48,601 POSTS
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.
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Tuesday, June 13th, 2017 AT 6:16 AM
Tiny
88
  • MEMBER
  • 59 POSTS
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.
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Tuesday, June 13th, 2017 AT 3:51 PM
Tiny
HMAC300
  • MECHANIC
  • 48,601 POSTS
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.
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Tuesday, June 13th, 2017 AT 4:04 PM
Tiny
88
  • MEMBER
  • 59 POSTS
Thanks HMAC300. I will check that out. You figure that alone could account for these symptoms?
88
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Tuesday, June 13th, 2017 AT 4:08 PM
Tiny
STEVE W.
  • MECHANIC
  • 13,560 POSTS
Pressure test the exhaust as well. Poor acceleration, running hot can easily be caused by a partly plugged exhaust, especially in an older vehicle.
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Tuesday, June 13th, 2017 AT 7:55 PM
Tiny
88
  • MEMBER
  • 59 POSTS
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
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Tuesday, June 13th, 2017 AT 9:16 PM
Tiny
SATURNTECH9
  • MECHANIC
  • 30,869 POSTS
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.
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Tuesday, June 13th, 2017 AT 10:26 PM
Tiny
STEVE W.
  • MECHANIC
  • 13,560 POSTS
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.
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Tuesday, June 13th, 2017 AT 10:30 PM
Tiny
SATURNTECH9
  • MECHANIC
  • 30,869 POSTS
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.
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Tuesday, June 13th, 2017 AT 10:30 PM
Tiny
SATURNTECH9
  • MECHANIC
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Be careful there can be super heated exhaust coming out the tail pipe.
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Tuesday, June 13th, 2017 AT 10:32 PM
Tiny
SATURNTECH9
  • MECHANIC
  • 30,869 POSTS
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.
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Tuesday, June 13th, 2017 AT 10:43 PM
Tiny
88
  • MEMBER
  • 59 POSTS
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
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Wednesday, June 14th, 2017 AT 1:44 AM
Tiny
SATURNTECH9
  • MECHANIC
  • 30,869 POSTS
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.
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Wednesday, June 14th, 2017 AT 1:28 PM
Tiny
RENEE L
  • ADMIN
  • 1,260 POSTS
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

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Wednesday, June 14th, 2017 AT 3:48 PM
Tiny
88
  • MEMBER
  • 59 POSTS
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
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Saturday, June 17th, 2017 AT 2:08 PM
Tiny
SATURNTECH9
  • MECHANIC
  • 30,869 POSTS
Usually not unless the core of it has broken loose and chunks of it moved and caused a clog.
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Saturday, June 17th, 2017 AT 8:16 PM
Tiny
88
  • MEMBER
  • 59 POSTS
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
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Thursday, June 22nd, 2017 AT 2:06 PM
Tiny
SATURNTECH9
  • MECHANIC
  • 30,869 POSTS
Why do you not just unhook the down pipes off of the exhaust manifolds and see if that makes a difference.
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Thursday, June 22nd, 2017 AT 11:43 PM
Tiny
88
  • MEMBER
  • 59 POSTS
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
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Friday, June 23rd, 2017 AT 12:20 AM
Tiny
SATURNTECH9
  • MECHANIC
  • 30,869 POSTS
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.
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Friday, June 23rd, 2017 AT 8:15 AM

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