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Old 04-08-2010, 09:04 PM
deus402 deus402 is offline
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Default 1985 GN high idle, no power. tried TPS and IAC procedures, but no luck.

I have a bone stock 85 Grand National. It idles around 1000rpm, and bogs under WOT. It has been like this or worse since i first got my hands on it.

First thing i did was checked the fuel pressure, and it is getting 60 psi at the rail at idle, i have no way to test it while driving because my gauge isn't long enough though.


the IAC counts were around 130, but now i have the minimum air screw (aka idle screw) backed all the way out to where it isn't even touching the throttle blade linkage, and the lowest i can get the IAC counts to is around 80.

After i adjusted the IAC the best i could, i reset the TPS to .45 at idle. I also pulled the EGR valve and made sure that it was clean and operational, since they tend to cause idle problems on GM's as well.

After i had all that set, i hooked up a vacuum/boost gauge, T'd into the MAP vacuum line, because the dash boost gauge shows it going to 15PSI almost instantly, which i knew couldn't be right.

took it out for a test spin, and noticed several things that seemed relevant. first, the engine bogs down when the pressure/vacuum goes to 0 psi/kPa (or maybe it goes to zero because it bogs... ) at idle i am around 20 in Hg / 50 cm Hg of vacuum, and see the vacuum drop (approach 0) as the throttle increases.

also, at WOT the O2 Sensor voltage drops to .1 V or less, sometimes 0, and stays there.


other notes:

my TPS seems to be working OK, shows .45 V at idle, and around 4.8V at WOT, which seems about dead on.

IAC counts jumps up 130+ under acceleration, but drop back down to around 80 when i let off again.

At idle, O2 counts are around 200, which seems like it might be a little high vs specs posted on the site, but i always understood that higher was better because it was reacting quicker.

I have also tried removing the fuel pressure regulator and plugging the vacuum line to get more fuel, but that really makes it run like crap at all rpm/throttle values.

did a QAD vacuum leak test with a can of ether, and nothing is immediately obvious by sight or sound either.

With a vacuum pump hooked up to the wastegate actuator, it does move, i didn't actually measure how much, but it seems like it's working right. however, it doesn't seem like the turbo is spooling up. it actually seems like the power cuts out right when the turbo should be starting to produce boost (however, i have never actually driven a turbo buick that ran right, so that is just speculation)

I can't quite wrap my head around what the actual problem is... not sure if my O2 readings are due to a bad sensor, or because of fuel starvation actually causing that lean of a condition. also, i'm not quite sure why i can't get my IAC counts into spec. possibly related to the O2 problem? maybe something else?

right now i am halfway into removing the throttle body to inspect the turbine and shaft to make sure it isn't seized/bent/otherwise hosed, and i have the O2 sensor soaking in PB blaster so i can bench test it with a torch. any other ideas would be much appreciated, i'm running out quick.

Disclaimers: all sensor values are from winaldl, using a mastach (mastach.com) cable. the actual prom isn't listed, but all numbers and codes "make sense." my IAC was replaced with a junkyard one, i just grabbed 5 or 6 off of various mid to late 80's PFI 3.8 gm cars. my original was completely shot, as well as the first two from the boneyard that i tried. also, sorry in advance for the rambling style of writing, lots of info to relay, and i'm not the best wordsmith...

Last edited by deus402; 04-08-2010 at 09:17 PM. Reason: minor edit
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Old 04-08-2010, 11:55 PM
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Lightbulb Re: 1985 GN high idle, no power. tried TPS and IAC procedures, but no luck.

Well...

First, I'm a bit suspect of your scan tool. Some of that data sounds...well...off. Of course, you have said the car doesn't run quite right...

Personally, I would not hesitate to step up and spring for a fresh O2 sensor or EGR. Using used ones is a bit like grabbing...oh...a used tampon or something. (Sorry for that graphic image...*shudder*.)

I don't quite understand how you had the fuel pressure regulator removed and had the car running. I can see having the vacuum line off, but the regulator? Doesn't seem possible.

The IAC/TPS adjustment is pretty critical. Their adjustment is pretty delicate, and something tells me you aren't even in the ballpark.

Right now I think you need (1) to verify your scan tool data (2) gain access to a fuel pressure gauge that is readable while you are driving the car (3) insure an appropriate O2, fuel pressure, and boost reading while at WOT and (4) insure proper idle parameters.
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Old 04-09-2010, 03:56 AM
deus402 deus402 is offline
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Default Re: 1985 GN high idle, no power. tried TPS and IAC procedures, but no luck.

hmm, bad wording. i meant the i uplugged the vacuum line from the fuel pressure regulator.

i've been digging through the site, and one thing i noticed was in this post:

http://www.gnttype.org/forums/showpo...4&postcount=10

it mentions "regardless of how you scan the computer, you do not want the 10k mode of the scanner working. Or with other scanners, you want the car in "test drive" mode when scanning"

which isn't mentioned here:

http://www.gnttype.org/techarea/faq/IACresetscan.html

winaldl only supports the 10k mode, so i'm going to try again with the paperclip method tomorrow.

i'm still going to pull the turbo and give it a quick once over, and bench test the O2 as planned, just to eliminate possible points of failure.

also, as far as the junkyard parts, it was either spend an hour at the junkyard collecting the correct IAC motors, paying $4 for 6 of them, and having them today to replace my known bad one, or paying $70-$80 for one, having to order it, and not be able to continue diagnosis until next week whenever it showed up. I am still waiting on bumper fillers and an air filter that i ordered the day i got the car...


as far as the points that you addressed....

1) I honestly have no reliable way of verifying further than i already have, short of buying a scan tool... winaldl gives you access to raw data as well as converted, i.e. tps at .45V and 0%, which i can verify data like that by running my meter across the wires (which i have done) I can also do the same for other sensors as well, but for instance, the IAC counts cannot be verified without comparing to another scan tool. however, like i said, all the data seems to make sense, i.e., it may not be in range, but when i move the throttle, the tps voltage changes. when i mess with the air screw the iac counts changes in the expected direction etc. also, i have used winaldl extensively for troubleshooting other gm vehicles with great success. i completely understand the need for accurate data for accurate diagnosis, but i am somewhere in between i trust what it's telling me, and i have no choice but to trust what it's telling me... FWIW, i also installed turbo meter and the data it displays coincides with what winaldl displays.

2) I concur, my gauge has always frustrated me for this exact reason. i'm thinking i might be able to find the fittings at a hardware store to make an extension for it, and then just get a length of high pressure fuel line to attach them to.

3) the fuel pressure is the only one i'm not sure is not appropriate. i assume the map sensor line was a good place to T into for my boost gauge, both because i am ostensibly seeing the same pressures as the ecm, and its close enough i can hang my gauge through the passenger window while driving. according to my gauge i never hit positive pressure... as soon as it comes up to 0 it bogs, which is why i'm going to check the turbo condition... i'm going to bench test the O2 tomorrow, and i may just bite the bullet and buy a new O2 sensor, but i have about as much faith in a new autozone sensor working as i do the 25 year old original one. I kind of hope that the O2 just fails the bench test, but i don't think that the problem is isolated to just the O2 either. (due to lack of boost)

4) back to what i said about how i tried setting it in the wrong mode. not sure how much it will help setting it with just the paperclip, since i already have the air screw backed all the way out, but i'm also not sure what having the 10k mode enable is doing to the IAC either. i suppose it's also possible that a bad O2 sensor is hosing everthing i'm trying to do with the idle.

finally, i really appreciate the help. when i first picked up the GN i signed up at all 3 forums for turbo buicks, and your site by far has the most amount of information available, not to mention your willingness to help. I really do appreciate it.

Last edited by deus402; 04-09-2010 at 04:50 AM.
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Old 04-12-2010, 04:50 PM
deus402 deus402 is offline
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Default Re: 1985 GN high idle, no power. tried TPS and IAC procedures, but no luck.

Update: Turbo seems good. turbine side is nice and tight with no play in the shaft and the fins all look good, and it spins freely. Compressor side has just a touch of play in it, but doesn't seem to rub on the housing, and again the fins are all good and it turns freely. also i did the ghetto turbo test, and it passed (see here: http://www.youtube.com/watch?v=jOGYIsAizQU )

while i had the up-pipe (?) off, i did the "bench test" of the O2 sensor even though it was still in the car, and have determined that it is bad. i have a new one on the way from the parts store right now, and hopefully i should have the turbo and throttle body back on by the time it gets here. had to order an O2 socket as well, because there is no way to get the sensor out without one that i can determine.

still haven't got a fuel pressure gauge extension yet, but hopefully the O2 will get me on the right track without having to deal with that.

Also, If i understand correctly there is overboost protection... and my factory boost gauge is wildly inaccurate. it's more like an "are you touching the throttle" gauge. with only part throttle it jumps up to 15lbs of boost in under 2 seconds. (add on gauge never reads above 0...)

this is entirely based on speculation, but here's my theory:

factory boost gauge reads 15lbs, so overboost protection kicks in (i assume it activates the wastegate solenoid and just dumps all the boost, but i guess it could cut spark, timing, or fuel...) which is what is causing it to bog when it should be taking off like a rocket. so i guess my questions are:

1) is there overboost protection on an 85, or is it just on the 86-87 with the better ecm?

2) what is the factory boost guage getting it's reading from? MAP sensor i assume...

3) what, if it even exists, does overboost protection do to get the boost back to a safe level

4) am i even on the right track in assuming that this could be causing my symptoms?

Last edited by deus402; 04-12-2010 at 05:05 PM.
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Old 04-12-2010, 05:54 PM
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Lightbulb Re: 1985 GN high idle, no power. tried TPS and IAC procedures, but no luck.

I think you are being gracious in calling it "overboost protection"...

It is more like headgasket protection. The wastegate solenoid can only bleed about 5 PSI. After that, the ECM can then cut timing. Usually if detonation (i.e. knock) is sensed. Note that the MAP sensor has no connection to the ECM. None. Zero. Zip. Nada. The boost gauge is for your info only. So if you don't know any better, and think 15+ PSI at 5 MPH is fine, enjoy...til it pops.

I suspect that the wastegate is either not working right, or the hoses to it are plumbed right. If the wastegate solenoid is bad, it likely would not increase so fast. Since it can only make minimal adjustments, I don't think it would cause this.

Make sure the routing of the hoses is correct. Pretty sure there is diagram on the site.
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