My '85 carburated 305 has suddenly lost power and will run hot if I push it more than 50 mph. I suspected retarded timing but found base timing to be only 1 or 2 degrees retarded - doesn't seem like enough to cause this. I can see electronic advance working at idle speeds running about 15 degrees advanced. No dragging brakes. Low power occurs with engine at normal or below temperature. I believe problem causing low power is also causing overheating. It does not appear that overheating is causing low power. Cooling system seems ok - coolant level is good and runs under pressure.

No computer error codes. Oil has 1700 miles on it. ATF level normal. Rear axle gear lube normal.

Next on check list is remove accesory drive belts and look for dragging component. Maybe I need an oil change? Any other ideas?

 

How's the carbs state of tune? How do your plugs look?

 

I don't do any tuning on the quadrajet except lube the linkage. What are you thinking with the carb?

As for plugs they are less than a year old, have about 10k miles on them. The engine runs smooth, no hint of misfire.

I want to add that I can feel the engine starting to cut out at 2500 to 3000 rpm. But the low power and running hot occurs belowthat rpm...

 

Could be a clogged exhaust, check backpressure or via intake vacuum..if checking with vacuum compare idle vacuum to vacuum at 3000rpms, if it starts dropping from idle reading, you're clogged/clogging.

 

Clogged exhaust? That's a new one on me! OK thanks for the tip. I will have to wait till next weekend to pursue...

 

i've noticed something like this too. you say clogged exhaust, do you mean the cat? its the same one as when i bought the car in november.

thanks,
brandon

 

Yeah, I'd say plugged CAT as the most likely cause, or a kinked pipe -or take a look and make sure you have good circulation from your water pump. Impellars wear out, or come loose from the shaft. Seen it happen.

 

Thanks for the replies everybody. My cooling system seems ok, but you're right an eroded impeller could cause the overheating. But remember I am saying the low power occurs at any temperature and the overheating happens when I try to push the car over 50 or so.

Yes we are talking clogged catalytic converter here. This car is 19 years old and i don't know if I have the original cat or not. It looks old. I won't be able to work on the car until Friday so I have done a little research. This link http://www.misterfixit.com/cat.htm describes the rev limiting effect a clogged cat can have. This other link http://members.aol.com/carleyware/library/overheat.htm talks about over heating.

So on Friday I will either purchase a vacuum gauge and do the measurments folks have described, or just yank off the cat and see if the car runs better. I'll let ya know...

 

any new word?

 

Hey tobias05 I'm back home and ready to dive into the clogged cat problem. I bought a vacuum just to do the check one of the early contributers suggested. I have to put the belts back on then I'll run the vacuum check. Talk to ya later...

 

For anyone who's seen my replies before you know I'm not a huge fan of emmisions equipment, so...

the best solution to a clogged cat is a straight peice of pipe. I did the to my 87 carbed Camaro and man did it sound good, you do lose a little back pressure but the sound is worth it, and if your cat is clogged anyway, your losing a lot of power so you should see gains.

My final solution to all carburated car owners is to remove the smog pump, AIR, EGR, and Cat (or at least go high flow).

 

Thanks scorch - I have seen several comments here on going with the straight pipe. But this is my daily driver and I need to pass the NY state inspection which requires everything in place. Now nothing has to work because here in Branchport we don't do a sniffer test, at least on the '85 models. So whatever I have on my car at the yearly inspection will at least look like a catalytic converter and function air pump...

 

I ran the vacuum test described earlier in this thread. Here are the results:

Vacuum (Inches Hg)

RPM 1000 1500 2000 2500 3000 3500

Cat in 17 17 15 14 9 --
place

Cat 20 20 20 19 17 --
disconnected
from exhaust

Cat 20 21 21 21 20 20
removed

With the cat in place there is a definate fall off in manifold vacuum.

In the second row I had disconnected the output end of the cat from the exhaust system thus removing the effect of muffler from the test. I was surprised to see manifold vacuum level out so much, but there is still a fall off at 2500 and 3000 rpm. I wish I had grabbed a reading a 3500 rpm. The leveling out of the vacuum shows that the muffler contributes significantly to the back pressure.

I removed the cat from the crossover producing an open pipe. The vacuum readings are within 1 inch Hg from 1000 to 3500 rpm. Comparing this set of readings to the set where the cat is still in place shows that the cat does contribute to back pressure.

My plan is to insert a straight pipe to replace the cat until my next NYS inspection. I will take another set of reads to see what kind of back pressure we get from the muffler alone...

 

I tried to put my test results in a table - what a failure. OK, so I'll try to include a plot here...

 

Dude, you need a reality check...the "best solution" is not a straight pipe....the best solution is to replace the offending item and figure out why it clogged in the first place...

Why aren't you a fan of emissions?

You don't like clean air?
You'd rather a smoke belching rich running car pull up along side of you while your windows are down?

Please elighten me why emission controls are not a good thing.

 

I'd venture to guess the reasons...

Your cat clogged and is breaking apart...clogging up the muffler.

Now that you know what the issue is...find out the cause...most likely running rich....no use replacing exhaust every xx-miles...find out why it clogged in the first place.

 

Oh, I didn't say emmision controls aren't a good thing I just prefer not to have them on my car... By the way if your car is properly tuned it will still pass inspection or at least mine did. With some simple ignition upgrades, the only emmision device my car still had was a high flow cat, and I still passed. For me it's not about how many things are on your car that GM put on there because some tree hugger complained about dirty air. As long as my vehicle is below the required emmision specs I'm happy with it. Well, it also must run well. I DO NOT approve of people stripping emmisions off there car to "save weight" or whatever, sorry that 50lbs isn't going to make a difference if your car runs like crap. It's all about tuning.

Finally, if some car pulled up next to me with my windows down I don't care how much crap it's putting into the air if the car can put down a 10 second quarter mile or better. When I say I'm not a fan of emmisions I mean the equipment itself not that I don't like clean air. I'm sorry if I offended anyone with my statement but I simply think, especially on older vehicles, some of this equipment not only makes the vehicle run poorly but also makes more pollution.

 

I replaced the cat with a straight pipe, reconnected the exhaust system and took another set of manifold vacuum readings. Very close to the open pipe readings. See attached plot. I then took a test drive and original power is regained with no overheating.

Examining the cat I find on the inlet end two broken up sections of the core material. It looks like a jumble of irregular charcoal lumps. On the exit end the core material is completly intact. So I doubt there is any blockage down stream from cat core pieces. Just to be sure I attached a shop vac to the inlet of the intermediate exhaust pipe to try and suck out anything that might be clogging. I only captured about a teaspoon of grit.

You folks called it right when you mentioned clogged exhaust. But the only clog appears to have been in the cat. As for the initial cause I do not know. There is no evidence of running rich. But I did recently replace my 3.08 rear end with a 3.73. So the engine has been running 20% higher revs. Thanks again...