Glowing Exhaust Manifolds 1992 454 TBI 7.4L P30 Motorhome

Hello, after reading a lot here i decided to join and ask my question with hope that some of you grearheads may have an answer. I drive a 1992 p30 chassis motorhome with a 7.4L 454 tbi in it. A while back I had a backfire and then loss of power. I brought the RV back to my friends' shop and if I remember correct we replaced the ignition module the ECM had fried and that was that. The car then seemed to run good until I went up a hill.

Long story short I rebuilt the motor with a new timing chain, lifters, (didn't touch the cam, it showed no wear), machined heads and new exhaust manifolds. I had the catalytic converter out and the muffler they are fine. There was no restriction. New spark plugs, rotor, cap, actually a new distributor, coil, map sensor, EGR was fine, new coolant sensor and oxygen sensor, started it up and it fired right up. We played with the timing at 4 degrees TDC, 8 and now it's at 12 and it is back to the way it was running. Good power in the city. Engine does not overheat it purrs. Now, when I go up a hill I still have the same issue! Both sides of the exhaust manifolds glow and if I give it more gas they just glow more. I noticed when I give the car a good amount of gas on a straight road for let's say 10 seconds, it's no problem and before you know it I am at 75 mph. If I do the same amount of gas going up hill, after ten seconds they glow.

I hooked up the shop's snapon computer again today and it didn't show any codes. I replaced the knock sensor also. When the car goes up a hill it does not overheat. Temperature is steady at 200 degrees and the clutch motor fan comes on as it should. Oh, I also put in a new thermostat. I replaced the fuel pump also because I had an extra one and I put a fuel pressure gauge in line. It reads steady at 10.5 psi (9-13 is spec.) By the way the MAP sensor tested out good so did the ignition coil.

I made friends with the shop foreman at the local GM dealership and he put his scanner on it and he says at idle it runs a little rich but not alarmingly much. I checked the engine ground and it's good. So is the battery ground. I rebuilt the fuel pressure regulator also. There are no leaks and all the vacuum lines have been replaced. The intake manifold has a steady vacuum reading. Everything was torqued to spec and the car runs great, idles great, and sounds like a killer chevy. Lastly, I took the injectors to a shop here and had them flow tested. This shop does it with the injectors sitting in the throttle body so I had to bring the whole thing to them. The test outcome was that they are both good and they both flow 510cc. Now I found an online calculator and that turns into almost 50pph. The part number on the injectors reads GM17084304RPD. Now, my online studies point toward the fact that they indeed are the correct part number for this vehicle (p30) but that these injectors should flow 75/80pph. Some say the fuel pump should be at a solid 13 and not between 9 & 13. So this is now overwhelmingly confusing. I learned a lot on this journey and it's now at a point where everyone I have asked about this is out of ideas.

The computer that is in it is a replacement one and it has no part number on it. it just shows this: NUMBER: 7-7060 and DC CODE: 1212GD The old prom was used and it says this on it: AWUJ. The gentleman at tbichips.com says that he can reprogram my prom and all of this stuff will go away but I am holding off on that because the vehicle wasn't doing this and I shouldn't have to modify things in order to get it back on track. I know I can do a lot of work and programming to make it better but I feel it shouldn't have to come to that. So there it is. I'd love to hear some suggestions from you folks. Thank you in advance for your time and knowledge.

Replace the EGR Valve. I had this same problem with my 454 chevy chassis Motorhome & pulled the heads & had the valves done, thinking I had a burned intake Valve! I still had the same problem after putting the heads back on! A leaking EGR Valve allows exhaust flame to get past the valve & ignite the fuel in the intake manifold. It only happens under hard acceleration, when there is a lot of pressure in the exhaust manifolds.

I'm sure other more knowledgeable people will chime in but i believe your timing should be set to 0 degrees with the brown wire unplugged. Also these systems are not super modern or sensitive so don't be alarmed if its not throwing codes.

Thank you for your reply. We had already tested the EGR valve and because you brought it up I just tested it again (20 seconds hold) and it holds vacuum.

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Originally Posted by joegreen

i believe your timing should be set to 0 degrees with the brown wire unplugged.

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Hello and thank you for your comment. The book says 4 degrees TDC and I have played with the timing several times. Timing is not the issue in this case! And last year, before all this, the engine was advanced 12 degrees which is where it's at now.

some one else had a post sometime ago about manifolds glowing, you might do a search at the top of the page and find what they did.

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Originally Posted by Kaweh

Thank you for your reply. We had already tested the EGR valve and because you brought it up I just tested it again (20 seconds hold) and it holds vacuum.

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What you did only tests the vacuum diaphram. The problem mentioned is the exhaust gas pressure opening the valve.

My '92 dually used to cruise with the headers glowing, I think the reason why GM stopped using iron exhaust manifolds and changed to steel header type manifolds is because the heat would crack the iron.
Another cause of backfiring is the screens on the distributor base getting clogged and not allowing any airflow, not that it needs much, but the screens need to be removed.

You can have the problem tuned out, or replace the whole eprom, that AWUJ code was updated, I don't remember with what. I put a BAMM code in my truck and it runs better, it has to run on at least 89 octane, thats the factory recommendation.

If the injector flow numbers are off why don't you correct that? The problem points almost directly at a fueling issue and you said you found one.

If (and its a big "if") the injectors you have are only flowing 510 cc (48.6 lb/hr) you have found your problem.

Possibly the flow is enough at idle and part throttle that it runs good , but when you load the engine (your uphill runs) it runs out of fuel causing the lean glowing red manifolds you describe.

A sure fire way to prove a fuel related problem would be to run the vehicle with the scanner connected and keep an eye on the O2 sensor reading , you will find out fast if it's going lean.

HTH

TOM

^X2 had that problem with a stock S10. not enough fuel in high load area

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Originally Posted by PJG1173

^X2 had that problem with a stock S10. not enough fuel in high load area

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^^ Which is why everyone that has TBI knowledge tells you that even a stock engine needs ATLEAST 13 psi to run correctly. I had a TBI L03 F-car that merely had shorty headers and exhaust on it and was running at 10.5 psi from the factory. The narrow band o2 sensor would go to ZERO at wide open throttle above about 3,000 rpm. Needless to say I understood very quickly why that engine got a bad reputation for not responding well to modifications. Bumped the pressure up to 14 and suddenly had a little fuel to play with. I

I will say to the OP what I said on Chevytalk.... 510cc is not enough fuel for a stock 170 hp 305, much less a 454. Its only enough fuel for about 160 HP at 85% duty cycle and .5 BSFC. If those injectors were used in a 454 it was at 28-32 PSI!

And to expand on what I said earlier , when your injectors were flowed , there had to be a pressure they were flow tested at .

We can assume the 510cc was flowed @ 10.5 psi (which is what GM originally flowed the 17084304 at) , but it does no good to assume . If it was in fact flowed @ 10.5 psi 510cc is not enough fuel .

If the injectors were flowed at a lower or higher pressure , flow will be proportionally different. Without a PSI for a given flow rate , the flow rate is useless .

FWIW , the 17084304 was flow rated by GM engineering @ 75 lb/hr @10.5 psi.

From an email message from Walt Sherwin dated 8/25/03:

"17084304 is the stock (1990 - 1993 454) SS injector. The service part number is 17112560. It is rated at 75 lbs/hr of spec fuel, at 10.5 psig differential pressure.

HTH

TOM

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Originally Posted by Nasty-Z

Without a PSI for a given flow rate , the flow rate is useless .


TOM

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The flow test was done at 13psi. Just found that out. I just wonder if the injectors are indeed the problem. And why did I have no problem all of last year?
How can I bump up the pressure of the fuel pump without going to something extreme like the ACDelco 381 pump (60psi)?

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Originally Posted by billygraves

Yes there are some things that can help. Without EGT and A/F ratio readings how is anyone suppose to help?

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What is EGT and A/F?
I use AC Delco when possible and available. I've done pretty much all of the labor myself and I have owned this vehicle for the past six years.

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Originally Posted by Kaweh

The flow test was done at 13psi. Just found that out. I just wonder if the injectors are indeed the problem. And why did I have no problem all of last year?
How can I bump up the pressure of the fuel pump without going to something extreme like the ACDelco 381 pump (60psi)?

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If the flow test was done @ 13 psi that means your injectors now flow 48.6 Lb/hr @ 13 psi , they are really flowing around 42.6 lb/hr @ the 10 psi you say you have ,not nearly enough to fuel your BBC under load , i'm actually surprised it runs as good as it does .

For those injectors to supply the fuel you would need around 35.5 psi at the injectors (this would give you a theoretical flow rate of 80.5 lb/hr) , this can be accomplished by an externally adjustable pressure regulator or modification to the factory regulator with either an aftermarket spring or the 94-95 454 TBI spring and a high flow pump such as the Walbro 255 LPH or a factory TPI or CPI pump. Only thing to remember is that this injector you have is rated from GM @ 75 lb/hr , for it to flow check as low as it did , something is wrong . I would not raise the pressure with these injectors.

If it were mine and I had hard data like that in front of me , I would get a set of flow matched injectors either on the 17084304 platform that you already have or the 5235231 platform injectors , both are around the same flow rate stock (75 lb/hr for the 17084304 and 80 lb/hr for the 5235231) . Mr injector is referenced on this board a lot although I have never had anything done by them , or if you could pick up another 454 TBI unit from a reputable source that has the injectors you need in it . Parts stores would be a last resort for me as you never know what you get unless it has flow rating @ stated PSI.

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Originally Posted by Kaweh

What is EGT and A/F?
I use AC Delco when possible and available. I've done pretty much all of the labor myself and I have owned this vehicle for the past six years.

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EGT is exhaust gas temperature and A/F is air / fuel.

Unless you have specialized gauges in your unit or the test equipment to check you would have no way of answering either without a substantial investment in both a Wideband gauge to monitor A/F and a Pyrometer to measure the EGT.


Hope this helps

TOM