2.5l stumbles at steady throttle...

I have an '86 Olds Calais Supreme with a 2.5 liter and a 5 speed. It has 180,000 miles on it. The car stumbles/bucks when cruising below roughly 2,000 RPM at steady throttle, not accelerating. IF I give it a little more
gas it will smooth out until it has settled in at a higher speed. If I accelerate enough to bring the RPM up above 2,000-2,500 it will run smoothly.
I have tried revving the engine in neutral, and get weird results. If I push the gas in a little to bring the RPM up to about 2,000 it will jump to 2,000 pretty quickly, run smooth for a second or two, then slowly drop to about 1,000 and run very rough/misfire. If I let go of the gas it drops to idle and idles pretty smoothly at 800-900 RPM (will slowly go up and down). Similar results if I give it gas to a higher RPM in neutral. It will go up to a given RPM, then start to stumble/misfire and slowly drop to about 1,000 RPM below where it initially went up to. If I give it a lot of gas in neutral to go to maybe 4,000 RPM it will stay up around 4,000 RPM and run smoothly.
When I got the car about 3,000 miles ago I gave it a tune-up which consisted of oil and filter, air filter, spark plugs, spark plug wires, distributor cap and rotor. I also put a bottle of fuel injector cleaner into the gas tank and cleaned up the throttle body as best I could without removing it by using throttle body cleaner. This greatly reduced the bucking/stumbling problem it had when I first got it. It wasn't bad at all after that and would only do it occasionally at particularly low RPM. I could cruise at 1,500 RPM and have no trouble.
The problem only got bad like I mentioned today after a session with my girlfriend driving, who is still in the process of learning how to drive a manual transmission, or shall I say, still learning finesse. ;) Anyway, as rush hour approached she got nervous about other cars and dragged the RPM down to 500 or so a few times before I told her to downshift, made several violent shifts, and towards the end stalled the car a lot, in a violent manner. The problem started after she stalled it three times in a row. She told me the check engine light came on. She stalled the car again seconds after that, and it didn't come on after she started the car, and hasn't come on since.
Right after that the problem got to the severity it is now. The car had all these symptoms before, but they were very minor, and I could even go a couple days without noticing any trouble. She was much harder on the car when she first started learning, and the car never gave any trouble. I don't really understand what could have happened to cause the current problem from her driving, so maybe it was just a coincidence.
Any ideas what the problem could be? All of my knowledge of cars is from my first two cars, a '67 Galaxie 500 and a '68 Galaxie 500, one had a 289 and the other a 302. I'll be driving my '68 again within a month or so when I get my new engine in, but I'm giving the Olds to my girlfriend so I would like it to be running properly. My only thoughts given my limited knowledge of modern fuel injected vehicles would be either a vacuum leak, malfunctioning IAC, malfunctioning EGR valve, malfunctioning throttle body, or dirty fuel filter. That is the order I suspect them in. Hopefully Thursday I'll have time to go through all that. Any thoughts or suggestions would be greatly appreciated. Thanks.
Cory
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push
2,000
1,000
consisted
by
come
all
don't
from
my
knowledge
body,
suggestions
Rey
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Is there any way to test that before I replace it? I noticed today that when revving in neutral even at high RPM (3,500-4,000) The RPM will fluctuate slightly (~100 RPM up and down) and it will be a little rough. Also, today the check engine light comes on within a minute after I start driving the car, and stays on until I next start the car.
Cory
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In the book I had, it lists possible causes for surges and stumbling holding throttle steady as:
Intake air leak Fuel pump or regulator faulty Fuel injection system Emissions control system
You can check the TPS and many other sensors by hand with a multimeter and small jumper wires. Oh, and the knowledge of what readings you should get.
If I remember correctly, that probably has throttle body injection, with just one injector, possibly two. And I believe those were prone to get pretty gunked up, and would only get really clean if cleaned by hand. But the possibilities extend far beyond that.
So, it's a process of elimination. Spark, oxygen, and fuel makes it happen, and you've taken care of spark.
Tony
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(TOP POST) It would help more if you can get the codes..... You can stand all day looking at sensors The code will tell you where to start then start checking you sensors..Also it will tell you if it is a sensor at all....May just be a misfire detect which is throwing off the light which could be you fuel system....to save time Id get the codes first ..Then we can tell you what to check... Steve C
Re: 2.5l stumbles at steady throttle... Group: alt.autos.gm Date: Thu, Mar 4, 2004, 11:06am From: snipped-for-privacy@webtv.net (Tony V.) In the book I had, it lists possible causes for surges and stumbling holding throttle steady as: Intake air leak Fuel pump or regulator faulty Fuel injection system Emissions control system You can check the TPS and many other sensors by hand with a multimeter and small jumper wires. Oh, and the knowledge of what readings you should get. If I remember correctly, that probably has throttle body injection, with just one injector, possibly two. And I believe those were prone to get pretty gunked up, and would only get really clean if cleaned by hand. But the possibilities extend far beyond that. So, it's a process of elimination. Spark, oxygen, and fuel makes it happen, and you've taken care of spark. Tony
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TOP POST To get the codes just with the ignition off use a jumper wire to terminals A and B which is the two terminal ion the top left....turn your ignition to run ...Do not try to crank it....you ses light will blink....it will flash once then pause then flash two more times letting you know your in diagnostic mode....one flash then pause the two flashes is code 12 which means diagnostic mode the just count the flashes then let it pause then count the ext flashes then there is you code let it flash untill the code 12 flashes again which means it is done and starting over the diagnosis process.....and then it will repeat the codes agian...turn your ignition off before unhooking the jumper wire to clear the codes just disconnect your nrg battery cable for about 30 sec then the codes will clear untill it detects another problem... Steve C Re: 2.5l stumbles at steady throttle... Group: alt.autos.gm Date: Thu, Mar 4, 2004, 11:06am From: snipped-for-privacy@webtv.net (Tony V.) In the book I had, it lists possible causes for surges and stumbling holding throttle steady as: Intake air leak Fuel pump or regulator faulty Fuel injection system Emissions control system You can check the TPS and many other sensors by hand with a multimeter and small jumper wires. Oh, and the knowledge of what readings you should get. If I remember correctly, that probably has throttle body injection, with just one injector, possibly two. And I believe those were prone to get pretty gunked up, and would only get really clean if cleaned by hand. But the possibilities extend far beyond that. So, it's a process of elimination. Spark, oxygen, and fuel makes it happen, and you've taken care of spark. Tony
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The problem was a vacuum leak. The EGR valve doesn't hold a perfect vacuum. If I suck on it with my mouth it moves a little, but slowly goes back. If I push the diaphagm in all the cay and hold my finger over the vacuum connection the valve slowly closes.
To test if that was the cause of the problem I blocked off the vacuum connection to the EGR valve and now teh car runs great. Took it for a drive up to Pep Boys to get a new EGR valve and it drove wonderfull, with lots of power for a little 4 banger. The onyl thing I noticed is that it would ping a little at low RPM if I gave it too much gas. Anyway, I got the new EGR valve but after I started to put it on I decided to test it to be sure it was good... Unfortunately it has a leaky diaphragm too. They aren't supposed to be like that are they?
In any case I've given up for the day. Perhaps tomorrow I'll exchange the EGR valve for a another one and see how that goes. For now it runs great without the EGR, though the check engine light comes on after a couple minutes of driving... Presumably because it's the exhaust O2 readings are out of the expected range. Oh well, as long as it works for now.
Cory
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Whoa, not so fast....

Quite possibly, they are. GM commonly used both positive back pressure and negative backpressure EGR valves, basically that means the EGRs operation is regulated by an internal backpressure transducer, IOWs, if yours is one of these, the vacuum decay is quite normal. The factory service manual will have the proper test procedure for the individual type of valve you may have.

I think you're on the right track suspecting the EGR valve, it is not uncommon for the internal transducer valving to become clogged which can create an 'over active' condition resulting in the symptoms you described.
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Oops, sorry, yes, checking codes would be priority number one. Several heads are better than one.
Tony
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The most likely cause is a bad EGR valve -- probably a pinhole leak in the diaphram. I chased this problem for weeks in an '80 Phoenix with the 2.5 engine. When you replace it, just be certain that the replacement is an exact duplicate for the one you took off. As another poster noted, GM used a couple of different configurations for their EGR valves.
Mark Sparge

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