93 K2500 Suburban 5.7 - stalls at idle

I just finished replacing the fuel pump (it was delivering only 4lbs) and it now is giving me a consistent 10lbs of pressure, and now at cruising I have no performance problems. I also replaced the cat and
the muffler with a straight pipe and muffler, and am awaiting a tailpipe I ordered to put on later this week. O2 sensor a foot ahead of the location of the cat is new-ish.
So now that some of the other problems have been corrected with the fuel pump, I'm getting a clearer picture of the remaining problem(s). When I first start it up cold without pressing the pedal, it starts quickly, but runs rough. After about 10-15 seconds, it smooths out and runs better - not perfect, but fairly good. I have the feeling it is running rich though. Lots of smoke from the exhaust initially until it warms up, heavy fuel smell. And it seems to run fine down the street. Then, when you stop at a light, the idle is okay for a moment, but then it often will quickly stumble and stall unless you juice the gas a little while it idles. This seems to happen when warm or cold.
After it is started and right as the idle smooths out somewhat, the check-engine light comes on. The code it is throwing is a 33 which my book says "Mass air flow (MAF) sensor - signal voltage or frequency is high during engine idle. OR Manifold absolute pressure (MAP) sensor - signal voltage is high during engine idle. (Note: Engine mis-fire or unstable idle may cause this code.)"
I'm told I don't have a MAF sensor, so I can rule that out. I replaced the MAP sensor before I changed the fuel pump, so it's not likely to be the cause, but I could throw the old one in to see if the problem changes. My thoughts are maybe fuel injectors or a vacuum leak. A while back I found a massive vacuum leak at the back of the TB where it mates to the manifold. I pulled it and re-did the gasket, but maybe it's returned. Engine has low miles (a replacement) and the truck has 230k miles, and most sensors have been replaced. Any ideas or suggestions on how to proceed?
Thanks in advance, --Jeff
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The thick gasket between TBI unit and intake manifold is know to fail with age. (I had one fail on my 89 several years ago) and cause it to run strange at times when it leaks. The MAP sensor would look out of range at idle when there was a leak and vacum and mixture was off based on what ECM was expecting to see. BTW, MAP sensor calibrates ECM to altitude at engine crank and based on this ECM know what to expect at idle as acceptable. A large vacum leak would through it out of range and mess idle mixtures up too. ----------------- TheSnoMan.com
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Start it up and spray some carb cleaner at where you think the leak is, see if idle changes, should at least give you a rough idea of if there is a leak or not
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Good tip. You need to spray it all around the base but it is a very thick gasket and I have seen them fail with age. ----------------- TheSnoMan.com
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Good tip, thanks. I was wondering how to detect vacuum leaks - I'll definitely carefully go through everything on top of the engine when I return to town in a week. I'm also going to check pulses on the timing light to make sure I'm not getting any extras or misses.
Now that I've driven it a little more, I'm starting to wonder if it's something else though - since it is running so rich. I would think that if there's a vacuum leak, it would run lean.
Thanks, --Jeff
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You are thinking carb here kinda. With fuelinjection.TBI when map detects lower pressure from leak it will increase fuel flow and the mixture will be uneven because of leak and cause it to run rich and uneven at times. . ----------------- TheSnoMan.com
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Higher pressure. That's why it's called a manifold _ABSOLUTE PRESSURE_ sensor.

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On Tue, 30 Oct 2007 20:57:32 -0500, aarcuda69062

I left "vacuum" out but then low "vacuum" pressure deferentail and high Absolute pressure is talking about same thing anyway in a engine. ----------------- TheSnoMan.com
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So what?

So what?
Your reply had it backwards.
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On Wed, 31 Oct 2007 18:32:07 -0500, aarcuda69062

Kind I guess but it was implied low vacuum pressure. ----------------- TheSnoMan.com
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No such thing.
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In article

Tee a vacuum gauge into the MAP sensor hose, verify that there is vacuum there when at idle.
High signal voltage can be caused by lack of vacuum signal to the fitting (no pneumatic shift when changing from KOEO to KOER), IOWs, a plugged vacuum port, it can also be caused by a bad sensor ground, so haul out the volt meter, backprobe the black wire at the MAP connector and measure the voltage between there and ground.
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Found a massive vac leak I should have seen before, but I've been too busy to get in there for a while. MAP sensor was not even plugged into vacuum at the TB. Plugged it in and the light went out and it ran pretty well.
Spraying around with carb-cleaner, I found that the idle changes a little when I spray around the EGR valve - does this valve pull air from outside at all? I could not isolate where the carb cleaner was being pulled in from exactly, but to rule out the EGR itself, I was wondering whether it pulled air in at all - perhaps from the "back" of it where there are holes in the disk. My gut tells me that it should not allow outside air in at all, but thought I'd check.
I also get a slight change when I spray carb-cleaner around the rear of the intake manifold. Again, can't isolate where exactly, but somewhere.
What's the recommended SOP in this situation? Just pull the intake and replace the gaskets? Any suggestions about which gaskets to purchase? Also whether to use goop of some sort or not.
Thanks, --Jeff
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If the intake is leaking I would replace the gasket. I use Felpro or the GM brand. Run a heavy bead of RTV across the front and rear of the engine lifter valley. Do not use the rubber gaskets, just RTV. Let the RTC set for 10 min or so before you go back with the intake. Use the gaskets left and right. I also use a product called fast tack to help hold the gaskets in place. Tighten the intake down working from the middle bolts out.
IF you do anything....make sure to change your oil and filter after the intake job. DO NOT OVERLOOK THIS STEP! Change the oil and filter before you start the engine.
I would replace the hard plastic vaccum lines that run from the egr solenoid with regular vaccum line. I had a recurring check engine light issue and this fixed it. You might also get a new vaccum line that feeds the MAP sensor.

Elbert snipped-for-privacy@me.com
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Exactly the information I was looking for, thanks. I notice that some lines (such as the one to the MAP sensor) use a harder plastic line rather than the usual soft rubber. Is it okay to replace this type with universal soft-rubber lines, or is the hard plastic used for some specific reason?
Changing oil - what's the reason for this? Is it in case you get dirt in the oil valley under the intake? Would it be beneficial to also pour a quart of cheap oil over your work area when finished to ensure that any dust is washed down to the pan? Of course, that's after taking necessary precautions to minimize the amount of stuff that drops in the first place and hitting it with a vacuum just in case.
Thanks, --Jeff
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I think the map sensor hose can still be bought new ( my friend at the GM shop put one on my truck) you could simply replace it with good "rubber" vaccum line. The plastic crap is prone to heat damage and becomes brittle and then leaks or just disintegrates. I would be tempted to get a new hose from GM on the MAP sensor. The EGR crap I would replace with regular vaccum line. Once I did that my check engine problems went away.
Changing the oil is important because you are likely to spill coolant in the oil when you do your intake swap.
I place a couple of rags in the lifter valley when I'm cleaning the heads of the old gasket. Once I get the heads clean the rags go in the trash or at least are removed from the engine. If you have a shop vaccum I would use that to remove any loose gasket material, I don't think I would introduce any foreign matter to the engine that you did not have to.
Don't skip the oil change after you are done, and prior to starting the engine. I'll be the best $20 you spend on parts all year long.
Oil change reduces the chance of coolant contaminating the oil and causes a potential issue with bearings... Don't skip this.
If you've never done an intake swap I would advise you to round up one of your friends to help who may have more experience. Its not a big deal but, I use to be in that line of work and have been screwing around with cars for a long time. If you are "new" then some things "we" may take for granted here, you may not be aware of.
I've not run through every step so you at least need to review a shop manual of some form prior to starting this if all new to you.
Don't use one of those scotch brite pads or other type of abrasive type pad to clean the gaskets surfaces.... the residue from these pads are known to cause problems with engines.

Elbert snipped-for-privacy@me.com
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