Boost = Hesitation

[dhoeschen]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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.