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Kaweh · 05-07-2015, 10:35 PM

Hi Ya'll,

I have 1991 P30 Chassis with 454 7.4L TBI Motorhome and after a back fire a while back that was caused by a short to ground from the previously modified injector harness, I noticed that the exhaust manifolds were glowing bright red when I was climbing a hill. I rebuilt the engine and replaced many parts and did several tests and it is still the case. Engine idles beautifully and drives very well. Only when climbing a hill, the manifolds start to glow in a matter of seconds and the glow goes all the way past the Y pipe.

Now, some say that this is normal and that perhaps I hadn't noticed it. I have owned this coach for six years now. I disagree with this statement of course because the air horns' line ran right next to the passenger side manifold and for the past six years the line was intact. Once I became aware of the glow, the air hose would melt. So I was wondering what experience some of you have with glowing manifolds when the engine is under a load?

It seems really safe to me. Thanks of your input!

Kaweh
www.kaweh.com
Spanish Guitarist San Francisco Bay Area

TeJay · 05-07-2015, 11:05 PM

I saw that one time during my years as a HS automotive instructor. It was on a mid engine car and I think it was a Pontiac Fierro sports car of some sort with a V-6. There will be others who will chime in on this but my recollection is that a lean air fuel mixture is usually the cause.

If it is a lean mixture that is not good because of the amount of heat. In the long run it can't be good for the valves.
The air fuel mixture at 14.7:1 bathes everything in fuel which acts to cool the combustion chambers. I'd do a google search on that topic and see what comes up.

It's been many years ago, probably mid 80's. I can't even remember what we did or didn't do to fix the issue. Now that I think about it I think it had a vacuum leak. The extra air was causing the mixture to get way to lean. An engine with a vacuum leak won't always run badly. After OBD II in 1996 went into effect the industry started monitoring and controlling more things. Having an OBD I vehicle there is not as much monitoring or control of the system. Yes they did try to reach and maintain a stoichiometric AFR of 14.7:1 but it was not as well controlled as it is today. Maybe that's why it seems to be running OK and no MIL light has lit up yet.

Something is not right and my guess is it a lean mixture or a vacuum leak causing the lean mixture.

This message is pushing my memory barrier way back and it's late at night. Since you did a rebuild you probably know a lot of this stuff so I hope it helps some. Again I'd do a google search. I'm sure there's guys out there who know a lot more than I do about it and would be or have already written about it.

One more thing that just popped into the head. We always used spray carb cleaner to find vacuum leaks. Spray the cleaner in suspected leaking areas, like intake manifold carb base gaskets, and the idle speed will change.

Do let us know how it turns out for you.

TeJay

britcon · 05-08-2015, 12:35 AM

I'd check for it being either too lean, or timing being off.

Skip426 · 05-08-2015, 01:42 AM

On your TBI engine it shouldn't be running lean, very long, without setting codes in the ECM and giving you a Check Engine light.
In a training film , when Dodge introduced the V-10 engine in Dodge trucks they showed a bare frame of a truck , on a chassis dyno, with the engine at 4,000 RPM under load , with the exhaust system glowing red , back to the cat convertors that were beside the drive shaft.
About the only way to test for a lean condition on TBI, would be to road test with a scan tool monitoring the O2 sensors and look for the sensors reacting by switching rich to lean , indicating the sensors are seeing the fuel injectors reacting to ECM signals to vary the pulse width and vary the fuel supply.

**JohnRR** · 05-08-2015, 02:38 AM

How far back does it get red like to the cat?

twinboat · 05-08-2015, 04:23 AM

A clue to your problem is the backfire. You should check for a exhaust restriction.

Hook up a vacuum gauge, and head up a hill and watch it.

At full throttle there should be no vacuum. If it starts to build, under heavy load, with the pedal to the floor, I would suspect an exhaust restriction.

Good luck

mrv8outboard · 05-08-2015, 06:22 AM

Retarded timing is one potential cause. The other is if this is a speed density system (no mass air meter) and a fresh rebuild broken in properly you could be leaning it out under heavy acceleration/load.

I had a 1988 Ford Bronco I towed my race boat with and every time we stopped for fuel at night and checked the oil the manifolds were just glowing red, not bright though. We were just exceeding the fuel map.

Now you say it back fired and now is having this symptom leads me to believe that your TBI base gasket has failed. this is a common failure on the TBI system.

M&EM · 05-08-2015, 08:13 AM

I have seen my pipes glow bright red under dyno tests done at Brazels, before and after install of their Ultra Power. I would guess that to be normal under full load conditions.

TeJay · 05-08-2015, 08:16 AM

He reported in Post #1 that the manifold is glowing past the "Y" pipe. Also because of this quote also from post #1:

"I disagree with this statement of course because the air horns' line ran right next to the passenger side manifold and for the past six years the line was intact. Once I became aware of the glow, the air hose would melt."

this does appear to be a new issue that did not happen before his rebuild.

Do check some of the possible causes: Blown base gasket (vacuum leak), exhaust obstruction. Since your exhaust has gotten very hot I have seen a piece of interior pipe fold over internally (which can't be seen from the outside) and caused an obstruction in the exhaust. Under normal driving it works OK but under extreme (hill climbing) conditions it can't get rid of the exhaust fast enough and things get very hot very quickly. The vacuum gauge test is a great point and a good way to determine back pressure occurring from an obstruction.

TeJay

M&EM · 05-08-2015, 09:22 AM

Actually he said: "Only when climbing a hill, the manifolds start to glow in a matter of seconds and the glow goes all the way past the Y pipe."

If it was a restriction he should have noticed loss of power under full load conditions. Perhaps a heat shield was lost during rebuild that protect associated horn stuff.

dieselclacker · 05-08-2015, 09:29 AM

Glowing manifolds after long climbs were normal occurrences on the two 454 powered motorhomes I owned. Easily observed at night thru front wheel openings immediately after stopping. Also noticed the same on 396 Chevy pickup I once owned. Pretty normal in that they all ran well for many years, and thousands of miles. These were carbureted engines, running stock jetting.

89sandman · 05-08-2015, 11:23 AM

When you set the timing did you disconnect the single brown wire with the black stripe? This has to be done to set the timing correctly if you didn't your timing is incorrect.

Kaweh · 05-08-2015, 11:31 AM

Thanks for all the info everyone. All of the things mentioned so far have been done and checked. I had the entire exhaust system apart and welded a new Catalytic converter in. The exhaust has no restrictions other than a bad design from he factory. This vehicle with this engine should have had dual exhausts.

I did another vacuum leak test by spraying a LOT of brake cleaner and we had the scan tool attached. there was no change in RPM or short term fuel trim. So the TBI gasket is ok. The heat shields were both lost. Don't ask why. lol but when the air horn line melted, they were intact.

Finally, my youtube video uploaded. It took all night long. it's a little bit shaky but I did collect some data.