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Discussion Starter #1
I am new here on The Diesel Place...got here by researching overheating. The LLY overheat saga sounds eerily similar to 1988-89 Chevy/GMC 7.4L TBI gasoline engines. Those early years of TBI big blocks ran too hot--exhaust manifolds glow red--and GM said it was for emissions. But 245-250 is too much heat, and the Turbo 400 trans doesn't like it either.
Anyone here experienced this? What did GM do to make the '90 and later
big blocks run cooler? They all have the same ECM. And one more thing
I have never got an answer to--why did the '88 454's not have a knock
sensor? The 350's did.
 

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Those year big blocks had a problem with the catalytic converters clogging, it was too close to the engine which made kitty overheat. Clogged kitty will OH engine quickly and turn manifolds a nice cherry red. Around the 90 model year the convertor was moved to just in front of the rear axle and mostly cured the engine OH problems.

Kitty was right next to tranny in those years.
 

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The catalytic converter has been removed from my truck and replaced with a straight pipe. Yes, it was right under the cab. The exhaust manifolds on my
truck will glow red even with the exhaust system open at the Y-pipe. I can light dry grass/weeds/hay on fire at tail pipe outlet if I have been driving at highway speeds for 10 minutes.
 

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What's the timing set at on your truck at idle and at 2500 rpms?

I just picked up a '95 K2500 Suburban with a 454 TBI motor in it (last year of the TBI). It says that at idle timing is only supposed to be 4* BTDC. That seems awfully low to me, I would expect around 10*, maybe better. If your exhaust manifolds glow red and the engine's running hot, that can frequently be an indication that the exhaust timing is heavily retarded (something that was frequently done for emissions purposes, but kills your power and mileage). Now, what I don't know is how they set that truck up in terms of ignition control. Is it computer-controlled distributor or does it still have the mechanical advance and vacuum advance on it? I would like to replace it with a complete distributorless ignition system. I bet you could really improve the power and economy that way. These motors have so much potential, but are then choked for emissions purposes. After all, it's a 7.4L motor making only 230 hp and 385 lb-ft.

Check your timing with a light, and try bumping it up a few degrees. You'll probably get better power, better economy, and your manifolds won't glow red.
 

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The advance curve is set by the ECM,you should time the engine according to the data plate.I would not advance it 6 degrees past where it supposed to be.Setting it that far advanced is harder on the starter motor,the rods,and it will ping and timing will be thrown way back by the knock sensor.Timing is usally checked with a single wire unplugged or terminal a-b jumped out,either way as soon as you reconnect the EST wire,the timing goes back to normal.If you want a more aggressive timing curve,get a tuned chip or downloader set up to run on premium fuel.
 

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Commissioner Gordon
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FordCrusherGT;1577905; said:
Is it computer-controlled distributor or does it still have the mechanical advance and vacuum advance on it?

:lol: Wow, there hasn't been mechanical or vacuum advance used since the mid '80s.

The low base timing is due to the computer controled timing. Most are 0 - 4*, after you pull the timing connector (read emission label). Then the computer can advance as it sees fit or til the knock sensor tells it to back off.
 

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One thing I forgot.... if you have access to a scanner then read what the o2 sensor voltage is... if showing rich then ECM will lean out mixture which will turn pipes red.

Set timing as per label on truck. A lot of folks set timing without unhooking wire or shorting aldl connector which leads to an actual base timing of around -14 degrees. IIRC there's a brown wire with a black stripe that needs to be unhooked to set timing,if so it's on the harness leading out of the relay cover, wire is outside of harness conduit.
 

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Discussion Starter #8
Local GM dealer has worked with me on this overheating 7.4L TBI engine.
Cooling system has been upgraded to '97 "tall" radiator for more capacity and higher efficiency (it's aluminum), by the way. We have tried various timing settings--adjusted by the book--but it just will not run well. Does it take the ECM some time/miles to learn the new setting, and if so, how much time/mileage? And the 1988 7.4L (and 2.5L, oddly) did not have knock sensors...confirmed by GM service manual.
 

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I didn't think that the truck would have mechanical or vacuum advance modules, that's just me being hopeful. :) I prefer either those or a fully distributorless setup with knock sensors. Anything in between is usually just annoying, as is being shown here.

This doesn't sound like a problem of a cooling system, this really sounds to me like an issue of the timing being too retarded. What you said initially about GM saying it was for emissions purposes backs my theory up. I have a mechanic friend who has retarded ignition timing to help cars pass tailpipe tests, and has said that in doing that the exhaust manifold will sometimes glow red. After the car has passed, the timing is returned to normal. The retarded timing lowers the overall IMEP (i.e. lowers cylinder temperatures and pressures) which produces fewer NOx emissions, and also reduces power/torque/fuel economy. The catch is that it will allow combustion to actually occur while the exhaust valve is open and the gasses are exiting the cylinder. This is dumb, you're just wasting all that energy that could be used for making your truck go.

If this is all computer-controlled timing and you can't adjust it at all, then I would actually go so far as to suggest finding a standard HEI distributor with vacuum and mechanical advance, and dump that in the truck. eBay should have something available fairly inexpensively.

The advanced timing I don't foresee causing any other problems, but I would be fairly conservative with it. I wouldn't want more than 10* BTDC at idle, and that's a maximum.

As far as you saying that you adjusted the timing by the book - what I have frequently found is that these computer-controlled distributors will not allow you to adjust the timing. You can move the distributor, but the other sensors will cause the spark to occur at the same time. However, this is just from my experience with this type of system. I have never actually worked with the exact system you are talking about in your truck.
 

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Commissioner Gordon
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That's why you pull the timing connector to set base timing. Doing this lets the ECM to drop down to base settings so the timing can be set where it is suppose to be. Once the connector is reconnected and the ignition cycled, all the ECM knows is where base should be set and calculates everything from there. If your off or try to do things like 'in the good old days' then the ECM doesn't know what's going on or how to recalibrate accordingly.
 

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Discussion Starter #11
In saying "by the book", I was meaning to say that the timing connector wire was dis-connected when setting the timing. And you are saying that the ECM
does not need time/miles to re-learn the new setting?
 

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Commissioner Gordon
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Nope. When you plug the connector back in and shut the truck down, let the ECM go to sleep and restart, everything should work as designed.
 

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stubblefeist;1580248; said:
In saying "by the book", I was meaning to say that the timing connector wire was dis-connected when setting the timing. And you are saying that the ECM
does not need time/miles to re-learn the new setting?
The ecm isnt aware of the new setting on an 88.The timing curve is fixed in the ECM,and the only way it will adjust is from the knock sensor(if it has one).It sets timing based on a number of input sensors including RPM,Load,(MAP) sensor,Throttle position,Coolant temp,intake air temp(some models),knock sensor(some models)which gear it is in(some models),and O2 sensor.The timing curve is set,and it isnt going to adjust for anything on those systems.Adjusting it requires you alter base timing,or change the program in the prom.
 
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