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Boost = Hesitation


dhoeschen
11-18-2010, 09:41 AM
1999 Pontiac GTP 85K miles, supercharged 3800 Series II.

The engine runs great, idles smooth about 18 inHG of vacuum at idle, no trouble codes, SES light is off.

While driving whenever a little more acceleration is needed, enough to allow the boost from the supercharger to come in, the engine can't take it and spits and sputters. When this happens I immediatly let off to not do any damage. This problem used to be random and not as severe. Now it is all the time and much worse. The engine wants to go but not with the supercharger.

So far I've changed spark plugs, installed new DIS coil packs and plug wires, checked for vacuum leaks, changed the fuel filter and ran some Techtron fuel system cleaner through 2 tanks of 91 octane.

I'm going to test the exhaust backpressure and try running it with the MAF disconnected. Any other suggestions?

dhoeschen
11-18-2010, 11:40 AM
I took a short drive with the MAF disconnected. Same acceleration issue, bad surging idle. Reconnected.

Looked in the manual for fuses and relays - the book says there are two fuel pump relays? I didn't have a chance to check under the hood yet - is there a high/low setting for the fuel pump? How about fuel pressure regulator?

rkvons
11-18-2010, 12:34 PM
The experts will jump onboard soon and tell you what to do. But I think the fuel pressure is a good place to start. You can disconnect the vacuum line to the fuel pressure regulator and see if there is gas in it. That would indicate a bad regulator. Also, you could put a fuel pressure gauge on the fuel rail and see how it acts. I believe during wide open throttle the fuel pump is supposed to kick into high gear. There is also a possibility of a clogged cat converter.

dhoeschen
11-18-2010, 12:45 PM
rkvons, thanks for the reply...

Do you think I'll see low fuel pressure while idling or do you think I'll have to check the fuel pressure somehow while driving?

I should mention that I rarely ever have "the pedal-to-the-metal" so not much WOT time, the car (used to be) scary quick without too much throttle!

olopezm
11-18-2010, 01:06 PM
Did you used a good brand of spark plug wires? Not long ago I bought a set of "duralast" plug wires (had to learn the bad way, i know) and the results were catastrophic, the Overdrive wouldn't kick in and the MPG were very low...

About fuel pressure, it would really help driving with the gauge taped to the windshield, that way you'll know how it is reacting.


To test the Fuel Pressure Regulator and fuel pressure, take fuel pressure reading key on/engine off (should be 48-55psi). Start the car, at idle fuel pressure should drop 3-7 psi from the KOEO reading. Then disconnect the vacuum from the FPR and readings should spike up to or near KOEO readings again.

Have you checked the vacuum lines at the air intake? That may also cause weird drivability problems.

Best regards,

Oscar.

dhoeschen
11-18-2010, 01:24 PM
Thanks olopezm!

Yes the spark plug wires are high quality MSD Super Conductor wires with dielectric grease on all the terminals. The plugs are OEM Autolite. I had some e3 plugs in before - no difference.

I have had to repair/replace most of the vacuum lines because nearly everything rubber is dried out and cracked. I will check again!

I have a vacuum gauge mounted in the car... wouldn't a vacuum leak show up on the gauge? I consistantly get right under 20 inHg at idle and 22+ on decel.

This problem popped up like flicking a switch, last weekend I installed a new HVAC blower motor and Monday morning I have no blower (supercharger). Ha! Ha! Stupid joke I know...

One more thought: I can hold the car in gear and keep the throttle "under" the boost threshold and it goes and goes! What changes (other than boost) when the supercharger kicks in?

olopezm
11-18-2010, 01:48 PM
OK so spark plugs and wires are ruled out. The vacuum reading seems to be OK normal range is 18 to 22 inHg and that could also give you an indicator of backpressure. If the vacuum is lower or keeps dropping more than usual then that would be an indicative of a plugged cat converter. However if you still want to check for backpressure that would be better.

Your problem might just be a coincidence between both things...

To be honest I'm not very familiar and obviously not an expert with superchargers, my GP doesn't has one. BUT as far as I know if the supercharger is dragging more air into the intake and the amount of fuel in the chambers is not the right one then your call will stall as the air to fuel ratio is not the correct one causing a lean mixture. So after that reasoning, either a fuel injector or fuel delivery problem should be the cause.

Hopefully one or more of the Grand Prix Masters in the forum will show up and give good advice about it.

Best regards,

Oscar.

dhoeschen
11-18-2010, 07:04 PM
Backpresure test results were normal. Less than 1 psi at idle and 2500 RPM.

I left the O2 sensor unplugged since it runs the same without it. Still no SES light - shouldn't it trip with the O2 sensor unplugged?

Can I bench test an O2 sensor?

Now if I can get a hold of a fuel pressure tester...

olopezm
11-18-2010, 07:22 PM
The O2 sensors don't trigger a code immediately, it takes about 3 driving cycles to trigger a code. You shouldn't leave it unplugged since each O2 sensor serves a purpose, specially the upstream sensors which monitor exhaust gases for rich or lean mixture so the PCM can make the proper adjustments; downstream sensors (after cat) monitor catalytic converter efficiency. These sensors rarely go bad and as another member said: a dtc regarding O2 sensors is only a starting point and not a full diagnostic.

I believe over there at the United States you can rent a fuel pressure tester at autozone or some other autoparts store.

Best regards,

Oscar.

olopezm
11-18-2010, 07:35 PM
I forgot to mention how to test the O2 sensor:

These usually work on the range form .1 to 1 volt, .1 means lean mixture and 1 volt means rich.

You have to get down there with the sensor plugged in, backprobe the connector (PLEASE BE CAREFUL WHILE JACKING THE CAR, USE JACKSTANDS) and check the readings.

A normal voltage should be .3-.5 volts
Unplug a vacuum hose and the value should change after a few moments.
If the value doesn't changes try unplugging another one, this shouldn't be necessary but just in case.
If remains unchanged, then you have a bad O2 sensor.
You can try to test for rich condition using some starter fluid, throttle body cleaner or propane, just be careful as fire may arise; have an assistant and a fire extinguisher near.

Best regards,

Oscar.

EDIT: Or you could just use a Live stream capable scanner.
Readings should be taken between pins A and C

dhoeschen
11-18-2010, 08:44 PM
You're right, there is no reason to not have it plugged in so I plugged it back in.

Thanks again for all the help! I'll check the voltage on the O2 sensor, the upstream one but probably not the one after the converter - unless you think I should.

I was able to borrow the backpressure gauge I'm checking to see if I can borrow a fuel pressure tester.

On a side note, on my way home from work I held it in 1st gear and part throttle (not hitting any boost) and it sweetly sang all the way up to 5500, forced a gear shift and wanted to keep going...

old_master
11-18-2010, 08:59 PM
Fuel pump should have battery voltage when cranking, (and when the supercharger is operating). After the engine is running, voltage is reduced to the pump which lowers the pressure. If you connect a fuel pressure tester, and disable the starter: turn the key to crank and watch pressure. Then turn the key to RUN, you should notice the pressure drop down a few psi, then drop slightly when the pump shuts off. GM has a bulletin out for the fuel pump bypass resistor. It changes the wiring and relocates the resistor to the firewall.

dhoeschen
11-19-2010, 08:50 AM
Thanks old_master, I think this has got to be a fuel issue, I should have a pressure tester later today so I can do some tests...

I'm also a little confused by your post... if the resistor was bad wouldn't that give me high pressure only and not any lower pressure? Plus I don't see a resistor listed on the RockAuto website (to see how much that costs) but that doesn't mean it doesn't exist. They do list this part that has a resistor: ACDELCO Part # D1780C {#15393413, 19116058, D1753A} RELAY,FUEL SHUTOFF SOL VLV 5-WAY, HI-PWR, W/RESISTOR, UNSHROUDED, BLACK, MICRO, CODE 7866

No idea what it looks like or where it goes...they call it a "fuel shutoff solenoid" but it sounds like a relay to me.

There were two fuel pump relays under the hood, I pulled them both out and switched them and put them back in. The vacuum connection to the fuel rail pressure regulator looked clean with no fuel in it.

What is the little do-dad downstream of the fuel regulator? Some little thingy that has a vacuum and an electrical connection - looks like it controls the boost bleed-off valve (or whatever that thing is) or maybe just sits next to it???

tblake
11-19-2010, 09:47 AM
Sorry I havn't been on here for like a week.

First check the vacuum line that goes into the lower intake manifold under the supercharger snout. There isa T-fitting there, follow the vacuum line to the back of the motor and make sure its still connected properly to the MAP sensor. Then follow it from the T fitting towards the front of the motor. it goes to the fuel pressure regulator and to the boost sensor. I have seen this line get cracked and cause all sorts of issues.

Next I would remove the supercharger belt and drive it like that. If the car runs better, your problem lies as you say with the boost system.

I suspect the boost bypass valve on this one as the culprit, but lets diagnose before you replace anything.

olopezm
11-19-2010, 10:06 AM
Good to hear from you Tim, I was only suggesting the normal tests since I have no knowledge on superchargers.

According to your post I guess the problem will be on the boost bypass valve you mentioned since he said to have replaced most of the vacuum lines. Will have to wait and confirm your suspicions.

Best regards,

Oscar.

dhoeschen
11-19-2010, 10:10 AM
Thanks tblake, I have had a lot of trouble with those vacuum fittings, every piece of rubber on this car is dry and rotten. I think I should replace them all just for piece of mind...

It will run without the supercharger belt!?! I'll give it a try...

tblake
11-19-2010, 11:51 AM
I ran mine a week without the supercharger belt as a test to see if it gets better milage with the belt on or off. Conclusion was it gets better with the belt. Plus the HP just wasn't there.

dhoeschen
11-19-2010, 12:12 PM
It sure drives better without the belt! It did have a few hiccups here and there when it needed to downshift or with a change in acceleration. Pretty minor compared to what it was doing before.

tblake
11-19-2010, 02:09 PM
Double check the vacuum lines on the top of the supercharger. There is a vacuum cap up there with 2 or 3 lines coming off it. One of those lines goes to the Boost Bypass Valve. Make sure that one is ok.

Bob/Rich/Lynn, what do you think? Does this sound like a bad Boost Bypass Valve?

Before you order one, lets see what the other guys have to say as I have not had much experience with them.

http://www.zzperformance.com/grand_prix/products1.php?id=840&catid=106

old_master
11-19-2010, 04:25 PM
I didn't mean to confuse you on the resistor deal. It's a fairly common issue, but you're right, if the resistor craps out you'll have no low speed in the pump. The engine will start and then stall after a about one second.

If you don't notice the hesitation with the belt removed, that could be an indication that the fuel pump high speed circuit has a problem. Fuel demand is greater when the supercharger kicks in and if the fuel pressure doesn't increase, it's gonna hesitate and power will be reduced. Check fuel pressure and listen to the pump: you should be able to hear the difference, as well as see it on the tester, when the pump changes speed from high to low and then off.

dhoeschen
11-19-2010, 06:25 PM
Did a quick test of the fuel pressure: 50psi when the key is on and about 47psi with the engine running. I pulled the vacuum line from the fuel pressure regulator and it is working as it should.

Replaced all the vacuum lines and it drives nicely and I get a little bit more vacuum on the gauge while driving...

The boost bypass valve is pulled up when the engine is running I'm going to test it with my vacuum pump to see if it has a leak... The other thingy right next to it (has a vacuum line from the fuel regulator and an electrical connection) has an open end on it - looks like a chunk of foam used to be glued on there at one point. Must be a reference to atmospheric pressure?

Need to test the high-speed circuit of the fuel pump...?

old_master
11-19-2010, 07:42 PM
Key on, engine off, fuel pump running: fuel pressure should be 48psi to 55psi. Engine running at idle pressure should drop approximately 3 to 10 psi. So it sounds like yours is ok, provided it remains above about 45psi for 10 minutes after the pump shuts off, (indicating leakdown is not excessive).

Do you have access to a digital volt ohm meter and are you comfortable using it? If so, there is a way to force the fuel pump to run at high speed and see if the hesitation goes away.

dhoeschen
11-19-2010, 09:42 PM
I re-did the fuel pressure test.

Key on, engine off, fuel pump running = 52 psi
Engine running = 45 psi
Engine off = 38 psi
Engine off after 7 minutes = 35 psi
Engine off after 10 minutes = 34 psi

tblake
11-20-2010, 09:27 AM
Fuel pressure looks good to me. But to be on the safe side, you could tape the guage to the windshield and take it for a spin. Make it do its thing and keep an eye on fuel pressure.

BNaylor
11-20-2010, 06:01 PM
Fuel pressure looks good to me too and not the problem. To the OP what is your boost indicator at the DIC reading with the supercharger hooked back up. Any specific rpm reading when the issue develops? What Autolite spark plugs did you install including heat range? Do you have a scan tool capable of reading out knock retard (KR)?

dhoeschen
11-21-2010, 01:09 PM
I'll be re-installing the supercharger belt sometime today and hopefully the roads will be free of ice so I can take a safe test drive... The fuel pressure gauge has a short hose on it so I may not be able to see anything.

I tested the boost bypass valve actuator; does this seem normal? I can pull and hold vacuum on the "upper" side which hooks into the main vacuum distribution on top but the "bottom" side does nothing...

Bnaylor, the spark plugs are OEM style Autolite #606 I don't know the heat range. The e3 plugs I pulled out looked normal in color. The DIC boost gauge will peg instantly. The problem happens at any speed or RPM when I push the throttle a little more which would require some boost. My scanner does not show KR or live data.

tblake
11-22-2010, 12:36 PM
Can you move the lever/boost bypass valve rod back and forth by hand? Will it move, and will it return back to where it was when you let go?

Sounding more and more like a BBP valve problem to me, but I have no experience with these. Mine is still original at 155k.

dhoeschen
11-22-2010, 12:55 PM
With the engine running the lever is all the way up, I can push on it and feel a little pressure. If I snap the throttle it will move downwards some and return to where it was.

tblake
11-22-2010, 01:55 PM
That sounds about right to me on the BBP valve.

Do you have any way to test exhaust system backpressure?

I've even in some cases removed the O2 sensor out of the manifold before the cat converter and drove the car like that to see if the problems were still present. Of course your car will be loud with the sensor out of the exhaust manifold.

dhoeschen
11-22-2010, 03:02 PM
Exhaust backpressure was one of the first things I tested. Less than 1 psi at idle and at 2500 RPM.

BNaylor
11-23-2010, 03:14 PM
Bnaylor, the spark plugs are OEM style Autolite #606 I don't know the heat range. The e3 plugs I pulled out looked normal in color. The DIC boost gauge will peg instantly. The problem happens at any speed or RPM when I push the throttle a little more which would require some boost. My scanner does not show KR or live data.

Do you have any mods especially like supercharger related? Using AL spark plugs is fine and 606 is equivalent to the OE stock from the factory AC Delco 41-921 or NGK PTR4B-15 double platinum spark plugs.

Head scratcher on this one. Maybe the KR is too high or kicking in due to the abnormal boost condition causing the reduction in power and the sputtering effect or in other words the PCM module is retarding ignition timing.

dhoeschen
11-23-2010, 03:53 PM
It's just a winter-beater grocery-getter...

It's got to be related to KR, I've tried everything else? I'm going to try resetting the ECM and see what happens...what the hell, I've tried/tested everything else!

Probally got a load of bad fuel; I remember filling my Tahoe with "premium" once and it was pinging like crazy. I went back to the gas station (why did I bother) and complained about paying for premium and getting something less, didn't help that the owner had no idea what the difference in octane was!!! He thought it was how "clean" the fuel was!

I believe I disconnect the battery for 30 minutes to reset the ECM?

olopezm
11-23-2010, 04:10 PM
I believe I disconnect the battery for 30 minutes to reset the ECM?

Sorry I can't help any further.

Yes, 30 minutes should be enough.

Best regards,

Oscar.

BNaylor
11-23-2010, 05:22 PM
Just to be sure on the boost control solenoid and valve does your scan tool have MAP readout capability? If you do there is an easy test. MAP (Manifold Absolute Pressure) value decreases while the boost solenoid is on. MAP value increases while boost solenoid is off. Or you can use a vacuum gauge at the boost signal hose.

The PCM module commands the boost control solenoid to operate at a 100% duty cycle (ON) for full boost pressure on demand. When there is rapid deceleration or the engine load is extremely high then the PCM will reduce boost pressure. Then the PCM commands the boost control solenoid to operate at a 0% duty cycle (OFF) which opens the bypass valve. When the bypass valve is open boost pressure is reduced.

dhoeschen
11-27-2010, 12:45 PM
Thanks BNaylor, my scan tool does not tell me anything other than errors. I normally work on non-computerized "muscle cars" and this car is just my winter-beater. :dunno: Perhaps in the future (warmer...) I'll troubleshoot into this further - here is my solution for now...

From what I can figure out, it seems like "premium" fuel around here is not what it used to be, octane may be a little low so I added a bottle of 104 octane boost to the tank. Did it help...??? Who knows...

Next, I may have made things worse on myself by changing out the e3 plugs in place of the Autolite 606 - even though this is what the guy behind the counter said works for my car - they don't. I checked the owners manual and found out what plugs I should be using and went a little "colder" with an Autolite XP104.

After taking a short test drive, I was able to get a little boost but still must have had some pinging (from what I've researched, the hesitation is from the computer sensing knock and retarding the timing) so I decided I'd rather have the car "drive" than slowly destroy its self so... I disabled the boost bypass valve and locked it in the open position.

The second test drive provided that my theory worked, although I don't have any boost I really don't care at this point - if someone offered me a reasonable price for the car I would take it and run away, far, far away... it's been that aggravating!

:banghead: My only hope is that perhaps the quality of fuel will increase and/or the ECU will figure out how much advance it should be adding - then maybe the car will drive like it used too... I'm out of ideas and have spent way too much time on this car - I still can't believe that one day it was fine and the next day sucked!

I'd like to thank everyone for offering their help, support and suggestions you all have really helped me through this and I'm grateful :) Thank you!

tblake
11-27-2010, 12:53 PM
What is a decent offer on the car, and do you have any pictures?

Its a bummer because I was just in cloud yesterday. I could have swung up with my powertuner and pulled up every engine sensor's data to see what exactly is happening.

Good call to go with the AL104's, although aren't the xp104's pricy? I'm just using the copper 104's in mine, but I replace them often.

tblake
11-27-2010, 12:55 PM
I will see if I can shoot a short video of my Boost bypass valve inoperation so you can compare. I still think your problem may lie with the bbv for some reason.

dhoeschen
11-27-2010, 02:28 PM
It's just a green 4-door not worth much in my opinion, I'll probably end up keeping it for a while... I don't want to think about making payments again! Don't get me wrong, It looks good from the outside but it has some hidden rust holes in the rocker panels under the plastic body panels. I'm concerned with the unibody integrity, the driver rear door has rubbed a spot inside the door - there is clearance at rest but obviously it's hitting/rubbing under driving conditions.

I'm just frustrated with it and probably won't give up quite yet - but I do need to step away from it for now. I have access to a big-ol' Snap-On diagnostic, perhaps I'll bring it over there one of these days.

Yes the XP104's cost me about $7 each, about the same I paid for the e3's.

I'm not sure about the BBV actuator. It seems to work, but I still have unanswered questions about the bottom vacuum hose connection... and that little electrical thingy afterwards. I drove it with that electrical thing unplugged - no change. I can hold vacuum on the top hose connection but nothing on the bottom.

tblake
11-27-2010, 08:09 PM
That sensor with the foam at the end of it is normal.

I think the BBV requires a large vacuum source, so I dont know if you used a manual hand pump to apply vacuum or how.

Like you say, when you start your car, the bbv will suck the valve open. Then if you feather the throttle, it will allow spring pressure to overcome vacuum and the rod will move down in correlation to how much throttle you give it.

ktrap
12-10-2010, 10:10 PM
OP any luck on the problem yet? I'm having some what if not the same thing, I have a 97 GTP with @ 180,000 miles on her. The instant the boost kicks in, boom hesitation...!! let of the gas for a second and let the boost drop back off and it's fine. I've had the sc belt of and it ran just fine?? So I feel your frustration..!!

dhoeschen
02-14-2011, 01:30 PM
I've been driving the car around for months with the boost valve tied open so there is no boost - and no problems. Stupid me took the tie off today to see if it may have fixed itself NOT!

Could this be a problem with the high-speed mode of the fuel pump? Like the high-speed mode is not "kicking-in" as it should? What can I do to force the pump into high-speed mode?

old_master
02-14-2011, 02:05 PM
This thread shows how to bypass the resistor: http://www.automotiveforums.com/vbulletin/showthread.php?t=658282

BNaylor
02-14-2011, 06:22 PM
This thread shows how to bypass the resistor: http://www.automotiveforums.com/vbulletin/showthread.php?t=658282

:thumbsup:



To the OP if pump high speed mode is out it will start and then stall within seconds or minutes but doesn't hurt to try the bypass procedure. In a worse case scenario based on your symptom the supercharger internal seals may be bad.

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