'87 Suburban, help diagnose engine problems (long)

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My 87 Chevy Suburban R20 (3/4 ton with all factory towing extras, gas 454 aka 7.3 L, TBI) has several perplexing engine problems. I have read in the newsgroup archives, but my head is spinning,
so I need more direct help.
My husband does most of the maintenance on this vehicle, but can't stand idiot mechanics. So I do that. I have tried to find a good mechanic (recommendations from other large vehicle owners, etc.), but so far no luck. In fact, I am getting really bad service (do they all think women are fair game?) I am in Santa Fe New Mexico. This vehicle travels at or above 7000 feet altitude, and usually tows 3500-4000 lbs of horse trailer plus horse and gear.
I bought the vehicle 10 months ago, and have put on 3,000 miles. Its 5-digit odometer showed 33,000 miles, and the previous owner (who had it several years and IIRC put on 20,000) asserted it was 133,000, so my title reads Not Actual Mileage. No idea if the true mileage is 133,000 or 233,000; wear and tear for an '87 suggests the former. I don't care which; the body has ~0 rust and is otherwise in very good condition, apart from the usual deferred minor maintenance. It always starts easily; never stalls, hesitates or surges. Automatic transmission, replaced with a rebuilt by the previous owner, shifts well. Now that we installed a locking gas cap (!), it gets about 7.5 mpg, mostly towing in mountains at highway speeds. We bought service manuals, including wiring manual, and DH reads them. Among much other maintenance, in that 10 months/3,000 miles DH (sometimes with me helping) has personally:
replaced air filter replaced PCV valve changed engine oil and filter (2x) added 1 quart oil (down 1 quart 1,000 miles after last oil change) changed transmission oil flushed cooling system replaced oxygen sensor (twice) replaced fuel filter (twice; second time by idiot mechanics) replaced spark plugs (twice) replaced spark plug wires (twice; first new set broke a wire!) replaced distributor rotor and cap replaced fuel pump and strainer replaced exhaust manifold gaskets and seals replaced speedometer (changed to a 6-digit odometer)
Now the problems
1. The speedometer and odometer both are inaccurate, by the same amount. At 40mph the meter reads 30mph; at 80mph it reads 40mph. Replacing the speedometer did not fix the problem. Is there a gear involved? How can I change it for the correct gear? This is an old problem. (And it means gas mileage is actually about 9mpg, not bad for a tow vehicle!)
2. The vehicle occasionally, nearing the crest of a long, steep hill, puts out some black exhaust *and/or* begins to lose power. Not nearly as much as diesel trucks the same size produce on a regular basis. I don't know if it normally has full power; it has enough that it can tow a trailer and keep up with traffic, but I have not driven any comparable vehicle at highway speeds. I think the black exhaust is tapering off, too; didn't see any on my last long trip. DH says the exhaust is rich (raw fuel), but his experience with the vehicle is limited to maintenance work, so it would be soon after starting the engine. I read the exhaust is supposed to be rich then. The 1st set of new spark plugs, in service only 500 miles, were gunked. Engine oil does not smell of gasoline, and looks okay. This is probably an old problem.
3. The Service Engine Soon light comes on within a few minutes of starting the engine, and mostly stays on. It sometimes goes off during trips, usually *after* climbing a steep hill. On my last trip, 4 hours each way, the light stayed off several hours at a time. DH used a paperclip to read the codes; the only code it gives indicates the O2 sensor complains of a lean mix. Changing the sensor the first time *may* have improved mileage, but hard to say, given that someone may have sucked small quantities of gas before the night they sucked 6 gallons (when I started the engine the next morning, the fuel guage *dropped*). This is an old problem.
4. Engine oil is now dripping from the back of the engine, where it joins the transmission. I think the leak had been slow, and has recently gotten faster. Up to a week or so ago, there never was an oil stain under the vehicle (probably the leak is worse when traveling), but now there is a fresh stain. This may be a new problem. This might account for the missing 1 quart, in which case it isn't going through the engine (yay!).
I have now taken the vehicle to 2 mechanics to diagnose the SES code. Shop #1 charged me for 2 hours of diagnostics and changed the fuel filter. No other information! The guy at the counter didn't know who did the work, or what the findings were.
Shop #2, which has the vehicle now, will charge me for 2 hours diagnostics, and I got a phonecall from the mechanic. He said the compression test showed low pressure in *some* cylinders. My Q: which ones? His A: well, all of them really. My Q: how about the valve seals and guides? His A: they're okay. And the problem is the rings (how does he know?), and he recommends I get a new (not rebuilt) engine. He'll do it for me for $6,000. This same guy took my service request at the counter and when I said the vehicle has 135,000+ miles on it he said "well there's your problem right there; old engine". So perhaps he thinks diagnostics are a waste of time; he already knows the answer.
So...what kind of information should I get for 2hr diagnostics?
Do I need another engine? If so, do I need a new one? I plan to keep this vehicle "forever", but even so it is 17 years old.
I have tried to disentangle these problems, but problems 2-4 may all be related. How can I tell? I guess we should fix problem 4 first, and see if I miss any more oil. And get the numbers from the compression test(s). About the O2 sensor; should we replace the catalytic converter?
Thanks,
    Una
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Please follow-up to alt.trucks.chevy alone. Thanks.
    Una
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First, the oil leak at the rear of the engine sounds like a rear main seal going bad. Before I would invest in any engine overhaul/replacement, I would try a few things that I know can work on some engines.
Buy a quart of Gunk 5-Minute engine flush and run it through the engine as per the instructions on the can. My 1990 Suburban 5.7L burned oil when I bought it and after using the flush, changing the oil and filter and diving it about 100 miles, it stopped burning oil and had more power (probably stuck rings). Run some Chevron Techron fuel treatment or Sea Foam fuel treatment through the system to see if it clears up any problems. These may or may not accomplish anything but they're worth a try since they're considerably cheaper than an engine.
****************************************** I would rather be exposed to the inconveniences attending too much liberty than to those attending too small a degree of it. ~ Thomas Jefferson ~
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main
on
engine
God man are you nuts?
My 1990 Suburban 5.7L burned oil

filter
power
I don't believe you.
Run some Chevron Techron fuel treatment or Sea

It will create problems.
These may or may not accomplish anything but they're worth a

Bullshit, they will cause an engines lube to fly right out the window, and where you gonna get your gaurantee then?
Hatt

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---snip to remove all irrelevant bullshit posted by DJ Hatt --
Oh big surprise there, there's nothing left. Were you born this stupid or did you have to work really, really hard at it? I stand by my original statements. I've never seen an engine problem created by using these products. The trick with any product is to READ AND FOLLOW ALL DIRECTIONS (I assume that you can read). The first rule of repairing anything is to go for the simplest (and cheapest) fix first.
****************************************** I would rather be exposed to the inconveniences attending too much liberty than to those attending too small a degree of it. ~ Thomas Jefferson ~
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---snip to remove all irrelevant bullshit posted by DJ Hatt --
Oh big surprise there, there's nothing left. Were you born this stupid or did you have to work really, really hard at it? I stand by my original statements. I've never seen an engine problem created by using these products. The trick with any product is to READ AND FOLLOW ALL DIRECTIONS (I assume that you can read). The first rule of repairing anything is to go for the simplest (and cheapest) fix first.
****************************************** I would rather be exposed to the inconveniences attending too much liberty than to those attending too small a degree of it. ~ Thomas Jefferson ~
I won't argue with your rationale, because it isn't rational. If you want to insult to make yourself look like someone who would use emotional control, you defeat yourself. So stand by whatever you want.
Hatt
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Una,
There are several mechanics that offer help to those who need it in this group. I am one of them.
One the other hand, I will not offer you any help. Why?,... because of the "idiot mechanic" remark.
You can take the suggestions of a novice, or you could get an honest response here from a mechanic. Which do you choose? I have been insulted. You won't get it from me.
My time is money, but not on this group. I like to solve problems. It's like a crossword puzzle to me. I log-on once a week to try and answer questions free of charge. Hmmm, ....I'm an idiot?
GMdude
P.S. Don't tell your mechanic that he is an idiot, he might just screw you on the repair bill.
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No insult intended. And I do see your point! Some people would use "idiot mechanic" to mean "idiot = mechanic". I don't use it that way, and you may choose not to take it that way. I hope so.
I choose advice from a mechanic who is not an idiot. Same for a physician, or a lawyer, or any other specialist. You would too, I am sure.
FWIW, in my profession diagnosis is an important skill, and I am good at it. So I know I have twice requested, been charged for, and paid for a diagnosis...and not gotten any. (I paid because, well, I learned something. Too bad what I learned was to take my business somewhere else.)
Peace?
    Una
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this
of
i would recommend getting a print out of the diagnostics. you did pay for it. as for your speedo, did that occur after a tire change? (tire size affects speedometer accuracy)
black smoke out the pipes does indicate rich mixture which should show up in the diagnostics. i'm not sure if you said you already replaced the cat converter. in a rich mixture, sometimes the cat converter will overheat (i've seen them glow red).
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wrote:

it in

because
honest
insulted.
pay for

affects
show up in

a rich

glow
Are you a mechanic? I thought it was a lean mixture is why I ask.
Hatt
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wrote:

did i get it reversed? carbon buildup in the exhaust is caused by lean mixture? i'm not an engine diagnostic tech. (electrical/steering&suspension/brakes/AC/cooling, about 15 years ago)
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Black smoke = rich no smoke = lean (somtimes will ping when really lean) and I remember a rotten egg smell from the exhaust.
wrote:

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DH and I discussed the lean/rich business at length, and this is the upshot. The error code indicates the O2 sensor reports a lean mix, i.e., too much O2. The exhaust smells rich, i.e., unburned gas. There are 2x2 = 4 possible combinations here:
Little O2 and all gas burned is the ideal, right?
Little O2 and unburned gas at the exhaust would mean there is too much gas going into the system but combustion is complete to the point the O2 is all used up.
Too much O2 and all gas burned means too little gas going into the system but combustion is complete.
Too much O2 and unburned gas (our situation) means combustion is not complete. The question is why?
Do I understand this lean/rich business correctly?
Wouldn't fouled spark plugs contribute to the poor combustion?
    Una
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The three Ts of combustion .... Turbulence, time, temperature. First make sure the engine is in good mechanical shape (leak down test / compression test). Now is a good time to "read" each of plugs to make sure they all roughly look uniform, see if you have a problem cylinder (this can throw off the good reading of the correctly burning cylinders). Run the engine while monitoring the voltage to all combustion chambers. Are all showing roughly the same values (in other words make sure you ignition wiring is performing correctly)? After these checks, you go on to engine management stuff. You need a good ECU / controls / sensor trouble shooting / wiring manual , a good digital meter and a scan tool. All reading should be with specified values. Next step is down 12 beers and give your head a rest...............
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Sounds like a typical failed O2 sensor to me. Sensor says lean and PCM adds fuel trying to drive the O2 rich. Result is a rich running engine with a lean code stored in the computer. Bob
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snipped-for-privacy@att.net (Una) wrote:

Let's see...
You have an 87 Suburban with a 454. The plugs are sooting up. The ECM is storing lean codes.
IIRC, that engine uses at least one, possibly two air pumps.
If the air pump switching valve(s) are malfunctioning and dumping air (raw O2) into the exhaust manifold when they shouldn't be (closed loop), you'll experience exactly what you're describing. Over fueled to the point where the piston rings no longer seat and the oil becomes so dilluted that the seals begin leaking...
I've seen this many times.
I've seen mechanics tripped up by it many times also.
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(Una) wrote:

Whats IIRC mean? The engine itself is essentially an air pump. In fact, thats really all it is if you want to reduce it to its simplest form so it can be repaird. I guess no one wants to, but they'd be a lot better off than applying theoretical rocket science with the all too often dubious, undocumented results, any where evident but for the bank account.
Hatt

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IIRC = "If I Recall Correctly" and he's referring to the A.I.R (Air Injection Reaction) pump(s).
-Bret
(Una) wrote:

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[I]f [I] [R]emember [C]orrectly
Ya see, since I'm not looking at this particular vehicle, I have to go off of memory. Being from Wisconsin and winters being what they are here with road salt, most of these shit boxes went to the boneyard years ago.

Duh. Would A.I.R (air injection reactor) have been clearer to you? (doubt it)

That's deep, very, very deep.

Was there theoretical rocket science in my reply? On air pump equiped engines, the air from the pump is diverted to the exhaust manifolds (exhaust ports actually) during cold engine operation to assist in reducing the pollutants after a cold start-up. Once the engine is warmed up sufficiently, or in this case, goes into closed loop fuel control, the air from the air pump is diverted to either the catalytic convertor or to the air cleaner housing (to muffle). It's not a great feat of logic that air being pumped into the exhaust ahead of the O2 sensor(s) when it shouldn't be is going to create a false lean indication to the ECM and the resultant rich mixture command by the ECM.
The real beauty is that it costs absolutely nothing to check to see if this is the cause of the erratic engine operation. Simply disconnect the air pumps temporarily and test drive the vehicle to see if the condition (code 44 with sooted plugs) returns.
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Neil;
It appears that we may be wasting our time on this one.
Regards,
G
wrote:

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