Help, Help
I can't for the life of me figure out why my cats start glowing red hot after running at idle for about 15minutes on the lift.
I am running a stroker 396 with AFR 220 cfm and a stout comp cam. Long Tubes to Dyna Tech High flow cats. (The little stainless steel jobs that are only 11" long and 4" in diameter.) thought it might be a programming issue but after three trys I think it might be something else. I have checked the valve lash over and over but don't seem to see anything pointing to a valve hanging open..... what could be causing this?!
Please help I am starting to loose my mind here

Thanks
Jason

 

What type of cats are they, and on what vehicle ? (I don't want to assume stock/highflow, and if it's a vette or not).

 

??? They are the Dyna Tech High flow universal fittments from Summit Racing. They are on a 1992 LT1-396cid stroker, afr 220cfm heads custom comp cam, long tube headers. running stock fuel pressure, using SVO red top injectors and a 250lph fuel pump. The intake is a ported LT4 and twin 58MM TB. Compression Ratio is 10.8:1, all forged bottom end. single mass fly wheel, internally balanced. Pistons and cobustion chambers are thermally coated and custom programming has been done.

I know that hot cats usually mean a rich condition but I don't see where the "rich" is coming from..... does anybody have an idea of what is causing this to happen??

 

What exactly IS your air fuel ratio? What makes you think 100% that it isn't running rich?

 

Umm unless you have another explaination as to why cats would get red hot I am positive it is running rich.
What I am trying to do is figure out why it is doing this. We have turned back the injector pulse width & tweaked the timing curve but no change with either adjustment or both together. I began to think that a valve might be hanging slightly open but after double checking the lash I don't see how one could be hanging open.... what I am stumped on is what is causing this beast to load up so much and so fast as to cause the cats to glow red hot while on a lift at idle.

open to ideas on this one

 

Does the engine have vacuum advance?

It needs low 20s to high 30s total idle timing, which is achieved from the combination of initial advance and full vacuum advance. The more valve overlap the more total idle advance the engine needs.

The vacuum advance should provide full advance at no more than 2" less than idle vacuum and be connected to a full time vacuum source, not ported. Not just any vacuum advance is correct. It has to matched the engine's idle vacuum characteristics.

If the total idle timing is correct the EGT is too cool for the cats to initiate oxidation reaction, even if the engine has air injection.

With no air injection a rich mixture won't overheat the cats because there is essentially no oxygen in the exhaust gas. So that takes you back to exessive EGT because total idle timing is insufficient.

Duke

 

So what you are saying is that with this LT1/4 style engine and the opti spark style distributor I need to program in more timing? Or should I be looking for some kind of a vacuum leak?

 

I don't have any experience with that system, but modern emission control spark advance maps are set up with much less than "ideal" timing at idle and low speed/low load. This increases EGT to keep the cats hot enough to promote reaction.

Assuming your system still has O2 sensors and idle mixture is maintained at stoich. the cats should not run red hot at idle.

Installing a high overlap cam on a modern integrated system can cause problems that are difficult to tune out. Your situation could be a combination of both mixture spark and timing issues.

That said, I have no idea how to change them on your vintage engine control system.

Duke

 

How about spark plug choice?
Does anyone think that heat range of the spark plugs could effect the EGT in this manner and thus cause the Cats to super heat as they are?

 

Some thing to look at..........
Rich fuel condition - fuel burning in your primaries. Can be caused by un-metered air being pulled in downstream of your MAF, dramatically retarded timing, low spark intensity (incomplete fuel burn), possibly a bad optispark, even a cam installed wrong.

Cats red hot: Rich fuel condition (fuel burning in cat). A rich fuel mixture can be caused by a header/exhaust leak sucking in fresh air, causing the O2 sensors to read a lean condition and adding fuel.

And also Tuning...........who is doing it?

 

Prom programming done by PCM for Less.


Do you think the AIR system may be malfunctioning? I really doubt that the opti is bad because other than the cats getting hot the thing runs like a top. If the opti was going bad I think it would be obvious (especially given my past experience with malfunctioning opti units)

What about my thoughts on the spark plug heat range? I know the plug itself doesn't generate heat but if I am running to hot of a plug then wouldn't that increase the EGT?

Then again if the AIR sytem is on while it doesn't need to be then the extra air could be a contributing factor right?

 

In response to your questions:

1. The AIR system isn't the source of the problem. It only injects enough air at the exhaust port to convert carbon monoxide into carbon dioxide and allow some partially burned hydrocarbons to finish the process after leaving the combustion chamber. Your problem sounds like fuel burning in the cats.

2. Heat ranges in spark plugs refer to the ability of the center electrode and the surrounding ceramic insulator to disipate heat. In general, "hotter" plugs retain more heat to burn off deposits in stop & go driving conditions while "colder" plugs tend to eliminate pre-ignition under full throttle conditions. Your red hot cats occurred at idle not at WOT so I doubt the heat range of the plugs is a problem.

Just taking a SWAG, I think that you have had an overrich condition that over time allowed unburned fuel to accumulate in the cats that generated enough heat to melt the converters honeycomb material to the point there was enough blockage to trap even more fuel as well as adding a tremedous amount of backpressure to the exhaust system. I would pull off both or your cats and check out the condition of the honeycomb support structure inside each one.

As to the source of the problem, I have found that fouled, misfiring spark plugs, weak spark from the distributor as well as leaky injectors all contributing to fuel burning in the cat(s). If there is a problem with your ignition advance, trapping your ECM data while idling the engine and checking the initial spark advance should resolve that question. It wouldn't hurt to re-check the injector data in the fuel map either.


Hope this helps.

 

Hey Greg,
The Cats are brand new high flow units from Dyna Tech. I will pull them and ensure they are still in good shape but I doubt I will find anything wrong with them to cause blockage as they have only seen a few minutes of use. something else must be the cuperate here.....


Jason

 

Are you still having problems with red hot cats? This can only be caused by raw fuel burning in the cats. What to look for is that you dont have any vacuum leaks first, then take a look at your mass air or map sensor and make sure they are functioning ok. borrow another and try to see if that helps. but the best thing to do is take a look at the timing and fuel calibration. If the timing is too soon, meaning the spark happens before top dead center not all the fuel is being burned off and its traveling into your cats to be reignited. Also if the idle fuel mixture is too rich due to calibration or a injector stuck open for what ever reason, or the fuel pressure is set too high or a bad regulator this will inject too much fuel for the cumbustion to be ignited. if you stand behind your car while its running and your eyes start to get irritated then your too rich for the idle circuit. Best cover all the basics and for the spark plug heat range, you drop 1 heat range for every 100 HP you added starting from the stock spark plug. remeber that spark plugs are used as a thermostat to pull heat away from the cumbustion to reduce emissions and primerily preignition.

Terry Chaitas
Rolling Thunderz

 

Jason,

From your last post, I take it that your had just installed the new cats when you noticed the problem while the Vette was on the lift. Is that correct? If so, did you happen to notice what the other cat(s) looked like inside once they were removed? (Just trying to figure out if this problem existed before the new cats or not.)

Along with the other problems mentioned above that would cause a rich condition resulting in unburned fuel turning the cats red hot, you may want to take a look at your idle fuel pressure. I just purchased an '89 Z51 out in St. Louis. While going over the car and the mods made by the previous owner, I found out that the adjustable fuel pressure had been lower to 32 psi in order to pass the local smog test. (The owner had replaced the stock 21 lb. injectors with 24 lb. injectors and chiped the car to run rich at WOT.) Turning the pressure back to the stock 36 psi at idle turned the exhaust black (too rich), so I turned it back down to 32 psi.

You might want to have PCM for Less take another look at your fuel map given the injector size and fuel pressure you're using for the engine. It may be that you were running just a little rich for safety's sake before whatever occured to make the Vette run super rich.

- Greg

 

Jason,
PM sent..............take heed

 

The cats that came off the last time met someone by the name of Black -n- Decker ...... so they flowed exceptionally well. I choose to reinstall these cats to make the VA state inspector happy. and now I am regretting leaving FL where they don't have state inspection

 

Thanks for the info Jason. Unfortunately, since there was no prior restriction or catalytic material to heat up, there is no way to determine if the condition existed before or not.

The installation of the new cats could have caused a leak(s) in the exhaust sytem as header bolts are routinely backed off to move headers to gain clearance, welds cracked as pipes were moved to gain access, etc. However, since the problem exists in both cats, I would strongly recommend looking at your fuel map/injector values along with the other information posted on this topic by forum members. Sorry I couldn't be of more help.

- Greg

 

Just watching and wondering about the outcome/cause.

Once had a car run rough on a 95 degree day. Pulled into a store parking lot, looked under cat was orange. Got out went in A/c and watched car from afar. LOL it was a Chevette. When I has a exaust leak many years later the guy took the cat off to show me the inside "once" had a meltdown and was gone.. Wonder when that happened.. He put a flange on it and put it back on... Man thoses days are gone.