Having some issues with our recently purchased car. Please help.

**mngtp** · 03-07-2011

We have a 1999 Grand Prix GTP(3.8l s/c) with 157,000 miles on it. We got this car for my wife. We both really like the car. I need some advice though. I have 3 questions:
1.)what could possibly cause the car not to warm up past the 1/4 mark on the temp gause? The heat in the car gets hot, but it does get any warmer than the 1/4 mark and then it drops to almost the lowest mark on the gauge. Is this normal? It seems to effect the fuel mileage a little when it drops.

2.)I replaced the spark plugs with AutoLite 606's (the copper core ones, I think those are the 606's). Like I read to do on hear. I noticed now the car doesnt produce as much boost at wide open throttle. Before the gauge would show full boost, but now it only goes to half when its wide open throttle.

3.)The SES (Service Engine Soon) light is on. Autozone said they cant read SES lights. Im not sure if that is true or if the guy was just lazy. I have been told that it could be something really simple setting the light off. Any thing I chould check first? Where else could I go to get the SES light code read (the dealership will probably charge a decent amount to read the light. I work at a Ford dealership as a detailer, I should know)?

Thanks in advance for the advice.

**Explicit_Spade** · 03-07-2011

1) A colder than stock thermostat and/ or cold weather could cause that
2) 606 are a factory temp plug, you may need to go a stage or two colder depending on your mods
3) Have a tech at the shop you work at pull the code when you have some free time. A ford dealer should have an obd2 code scanner.

**KeithGTP03** · 03-07-2011

lol...they cant read SES lights? Lazy bastid. As for the temp gauge, I'd agree with the colder thermostat, mine is almost always at the 1/4 mark. But did you say it drops down? If thats the case, I'd think it'd be the temp sensor. The code from the obd2 scanner should point you in the direction you need to go.

**mngtp** · 03-07-2011

1.)It does this on the freeway after I let it warm up. I was thinking the thermostat was maybe stuck open or something. Even idling it only goes to the mark just past the 1/4 mark on the temp gauge.
2.)The car is all stock accept for the free Thasher cold air intake. If I dont have any plans to mod it anytime soon, would the car be better off with different plugs next time they are changed? If so, which ones should I go with?

**KeithGTP03** · 03-07-2011

Alot of people here run copper plugs, but their cars are modded. I've been told by many people at the dealership that you will get bad gas mileage from copper plugs because they require more than our coils can put out, resulting in all the fuel not burning in the cylinder. I dont know how true this is, but the plugs wont be whats causing your issue. You need to pull the code and see what it is.

**mngtp** · 03-07-2011

I should have just gone with the irradiums.

even thought they were $7/plug. At least I know that the plug on the back side kind of suck to change now. Thanks for the advise. I will get the code read next time my wife lets me drive it to work. I usually drive our Cavalier to work since its got 208,000 miles on it, it gets about 35mpg on the freeway, we live 30 miles from my job and it runs on 87 octane not 91 or higher.

Does anyone know how to change the oil in the supercharger? I doubt that the oil has ever been changed.

**KeithGTP03** · 03-07-2011

I just changed to iridiums on Saturday, they are really simple to change out, it took me 50 minutes to do the plugs and wires. As for the s/c oil, there is a hex nut in the snout of the s/c (3/16 Allen wrench). Remove it and use something like a turkey baster or remove the top off of a bottle with a squeeze trigger. Stick the tube down in there and shoot all the oil into a container. You can get the s/c oil at the dealership. It will take 2 bottles GM p/n: 12345982

**Fivefingerdeathpunch** · 03-07-2011

copper core plugs will not give you less gas mileage

these boosted cars, just like copper cores better. I run them in my 99% stock GTP and I see zero difference in gas mileage.

As for you thermo. Get a new one (195* stock replacement) and see if it changes, If not your out 8 bucks, no big deal. Then start to look at the temp sensor.

As for your boost problem. Check your vacuum lines and see if any are broken or have come off of where they connect too. A broken line will cut your boost because it can leak out.

**Mike Kopstain** · 03-07-2011

Don't run copper plugs unless you need to. They need to be replaced more often and often are accompanied with a poor idle. I'd go back to double platinums if you have no intentions of modding.

As for the drop in boost, did that happen before or after the spark plug change? What about the check engine light? If it happened immediately after the plug change I'd suggest looking at the vacuum lines going to the boost bypass valve. It's possible you pulled the vacuum line off of the valve. Plugs themselves should have no bearing on boost.

**mngtp** · 03-07-2011

Are there any specific vac lines I should check out? I will have to look in the morning. The car still gets between 25-30mpg on the freeway even with the SES light on. Is that about right? just curious.

Im a little confused about the whole copper vs platinum vs irradium plug issue though. It sounds like its a mixed toppic

**KeithGTP03** · 03-07-2011

It is a mixed issue and a never ending battle. This is the vac line diagram. Check these lines for cracks or any sort of wear.

**Mike Kopstain** · 03-07-2011

In my opinion, it's not a mixed subject. :-) Run copper if you need copper. If you're not modified you don't need copper or colder plugs. There's no benefit to running them and a lot of reasons not to.

A good double platinum or iridium plug should be fine for you.

You're going to want to check the vacuum lines around the bypass valve. This is a little round, black plastic piece on the front of the car, just in front of the blower on the right side (if you're looking straight on at the car).

**FriboRage** · 03-08-2011

Just an FYI, I believe AutoZone and Advance Auto cant read "SVS" (Service Vehicle Soon) with OBDII scanners, you need a special scanner for it.

Which is why he might've said that, in confusion.

SVS lights usually indicate something like a burned out light bulb, I believe they also come up for ABS issues as well... Im not sure if our car has SVS, but my old Grand Am did.

But just go to Advance if you need a code read, they usually have great customer service in my experience, and I've been to several locations... free and fast too. I've found AutoZone isnt even worth walking in the door half the time.

Just come back here before you buy anything, if you're unsure about the code readout... dont let them talk you into buying stuff

**SlowNA06** · 03-08-2011

Originally Posted by mngtp

1.)what could possibly cause the car not to warm up past the 1/4 mark on the temp gause? The heat in the car gets hot, but it does get any warmer than the 1/4 mark and then it drops to almost the lowest mark on the gauge.

This could be a radiator cap. Or temp sensor. Maybe bad thermostat.

Originally Posted by mngtp

2.)I replaced the spark plugs with AutoLite 606's (the copper core ones, I think those are the 606's). Like I read to do on hear. I noticed now the car doesnt produce as much boost at wide open throttle. Before the gauge would show full boost, but now it only goes to half when its wide open throttle.

I bet you knocked a vacuum hose loose. Or broke that vacuum manifold on top of the supercharger leaning over to get the back plugs.

Coppers are only "necessary" if your pulley is smaller than the stock 3.8" IMO. Otherwise, use the stock $7 Iridium plugs (yes, GM switched to iridium, no longer using double platinums because iridiums are better). Coppers should be changed every 15,000 miles. A lot of people wait up to 30,000, but even an Autolite rep said they would fail as early as 16,000 miles in our engines. Iridiums should be changed every 100,000 miles and will work great for a stock engine. No plug will get you better mileage than any other. It's all the same fire, so long as you make sure you properly gapped your plugs before installing.

Originally Posted by mngtp

3.)The SES (Service Engine Soon) light is on. Autozone said they cant read SES lights. Im not sure if that is true or if the guy was just lazy.off.

Yeah, he was too dumb to work on your car. Try a different employee. My bet, leading back to my first answer, is that you'll see something like "front bank lean" when it's scanned. Also, +1 on Advance Auto Parts instead.

**mngtp** · 03-08-2011

Im at work. I just had the service manager read the SES code (he use t work at a GM dealership). It came up with P404 and P1404 codes. He said the codes were saying its a bad EGR valve. Is this true? He also said as far as the boost issue, the blower is going out. What do you guys think? Could the lack of boost be affected by a bad egr valve? I just looked on craigslist and I called John's auto parts (a junk yard here in minnesota) and a used supercharger is $200-$300. The egr valve is $150. I really hate those kind of suprises. I would really like your guys' feedback. You guys seem to know these cars best. Thanks.

**Mike Kopstain** · 03-09-2011

Originally Posted by mngtp

Im at work. I just had the service manager read the SES code (he use t work at a GM dealership). It came up with P404 and P1404 codes. He said the codes were saying its a bad EGR valve. Is this true? He also said as far as the boost issue, the blower is going out. What do you guys think? Could the lack of boost be affected by a bad egr valve? I just looked on craigslist and I called John's auto parts (a junk yard here in minnesota) and a used supercharger is $200-$300. The egr valve is $150. I really hate those kind of suprises. I would really like your guys' feedback. You guys seem to know these cars best. Thanks.

Your supercharger isn't going out. If you had full boost and then all of the sudden had half boost, you knocked a vacuum line off somewhere or damaged the bypass valve by accident. I've never one of these superchargers fail and immediately output less boost. If anything, they wear over time and probably lose a nominal amount of output. A "failing" blower would be one that is making loud bearing noises and even that blower would still make full boost.

**gtpinsc** · 03-08-2011

i didnt read all the posts but for half boost on boost gauge check all vac lines... especially the rubber T connection under the supercharger snout. u prob broke or knocked a line loose during plug and wire install. it happens to a lot of ppl.

copper plugs are fine.. i ran them bone stock and they performed well... just have to change them more often that iridiums and plats.

temp could be a few things.. bad temp sensor.. thermo is bad etc etc..

**dsmuts** · 03-08-2011

I doubt your sc is going bad. I'd pull off the egr and clean the hell out of it and see if that makes any difference, if not your not out anything but your time.

**KeithGTP03** · 03-08-2011

I scanned with my aeroforce today and was surprised (because my SES light isnt on) to see a p0403 and p0405 which are also EGR related. When I went to inspect it I saw that it wasnt there..lol. Apparently the previous owner deleted it. So first thing I'd check is if you still have one. If you do and you need to replace it, there are a bunch of people here that have deleted theirs and have it lying around and will sell it for a lot cheaper than brand new.

**mngtp** · 03-09-2011

I cleaned the EGR valve today but I will have to wait to see if it did anything until tomorrow. I tried looking but I didnt see any loose or unplugged vacume lines, One of the techs at my job told me to try spraying carb cleaner around the engine to help find the vacume leak. I did that but didnt find anything. Any other ideas on how to find a vacume leak? Is there anything else that could cause the loss of boost all of the sudden like this?

Thank you guys for helping me out and giving me advise.