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eric remington
eric remington,
Category: Pontiac
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Experience:  mechanic/service advisor at topline auto
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2006 Grand Prix 3800 Engine bucks under load when warm.

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2006 Grand Prix 3800Engine bucks under load when warm.Customer brought to me after a year of this issue. Other shops have replaced things without diagnosing. Crank sensor, cam sensor, coil packs, ignition module, 2 tune-ups (plugs and wires). MAF. MAP. Cat converter replaced. Freaking ECU was replaced with a new unit! I have replaced nothing myself (broke map and put another on though!).I started from scratch. Using maxisys.When first started, runs great. Acceleration is fine. After driving 7 miles or so, fully warmed up, the engine will buck. Sometimes violently. Road testing was no longer needed. Just warm it up and brake torque. I tested fuel pressure, 50PSI at all times, even when engine bucks hard.Vacuum does not go to pressure when bucking (hate to call it a misfire, since the computer does not register any misfiring).
The only vacuum tube to test at is the booster tube. So I ruled out a leaking booster while vacuum gauge hooked up.No sensors have any data out of specification or range. Broke MAP while pulling out, threw in a known good spare I have. (I have like, 20!).Fuel injectors not being cut out while bucking. Hooked up timing light and it goes dead while bucking. Figured the ignition is cutting out momentarily.Using lab scope, I find that the cam sensor and crank sensor signals are all good. They do not fall out when bucking. They just slow down a bit as expected with the RPMS dropping a bit.2 things I think are happening: lower intake manifold gaskets bad after warming up and metal expands, though no coolant loss. Could not pin-point with brake kleen being sprayed everywhere, but could be a leak from the valley instead, where you can't spray (aluminum intake BTW)Or maybe bad wire to ignition control? May need a hot-wire re-done to ICM?Throw everything you can at me. Looking for someone experience in the 3800 odd issues. DO NOT TAKE THIS QUESTION IF: you don't have devine knowledge of the GM 3800 platform. Looking for a seasoned 3800 tech. I already have 12 years experience myself, and this is an issue I never came across, and I think this is going to be a bad journey for the both of us! Will need help over the weekend. Will not need wiring diagrams, already have that. Just tell me your thoughts, what to test next, and I will get you the info.



Customer: replied 1 month ago.
A previous shop already ran it without the catalytic converter on it and it's still had the problem. However I will go ahead and pull the O2 sensor and run it and see if it's better.


Customer: replied 1 month ago.
Yeah, when I was loading it up it's still had just about a half inch of vacuum while it was bucking for over 5 Seconds. I really thought if I would have a plugged up cat that it would start building some pressure, but hey this is why I decided to go ahead and ask someone with Supreme knowledge you know what I mean? I will not be able to test this until way later on tonight.

WELL CONFIRM THAT AND LETS GO FROM THERE.....are map and maf new what kinda parts...they kind you buy in the alley? sorry I have to ask in diagnostics cannot assume have to assume everything was done wrong and that the parts were used etc know what I mean does it have supercharger? the lower intake manifold always go bad...I always replaced it when the upper plenum gets replaced...egr? what about egr? I will help you find I'm L1 is upper plenum? get me vin and I will look up tsbs

w​hat is engine vacuum? 18 and steady? do you have scanner? what is engine timing showing on scanner? so good fuel pressure? engine vacuum timing? at idle and under load? when warmed up...lemme know

vin number mileage

you can use those noid light type of spark testers to monitor spark

Customer: replied 1 month ago.
Just wanted to give you a quick update, I have had three of my customers come in with some emergency repairs. Since I work for myself, I have no one else to take care of them while I stay on this car. So I will get back with you later tonight or tomorrow morning with results of testing

k understood

Customer: replied 1 month ago.
The last three test drives half Ben without any engine bucking. Upon take off from a stop, and some times at speed if I'm going 35 miles an hour and I need to hit it, there is a slight hesitation around 1500 to 1800 RPMs but it is a blip. And then it goes right up all the way to 4500 RPMs.I will continue letting it cool down for maybe an hour or so and then retesting. Will update with any new information.
Customer: replied 1 month ago.
Alright I finally got this thing to screw up again! And now it's really bad to where I almost couldn't go down the road but when I switch over to neutral I can race engine right up to 4000 RPMs and even hold it for a couple seconds and it doesn't skip a beat! Only under load. I took out the O2 sensor while it was running, Upstream of the cat, and it made no difference. I also used a borescope to look down into the cat and everything looked fine, but either way having that hole before the catalytic converter did not help whatsoever.While I wait for a suggestion from you, I am going to do a couple voltage drop tests to see if perhaps there is excessive voltage drop before or after the ignition module. I will test from the battery straight into the power wire of the ignition module and get it to Buck around and then I will do the same thing for the ground side, Pins K&P are the power and grounds, I do have a print out of all that.
Customer: replied 1 month ago.
Also, I will check through my scan tool and then threw my lab scope how the knock sensors are acting. I figure if the knock sensors or even just one of them aren't helping the timing retard, maybe this whole thing is a pinging issue with being Advance far too much without retardation.
Customer: replied 1 month ago.
Alright, after doing some searching around I found that the HDM relay is a known issue. It has caused extreme failures and some cars, leading to melted fuse boxes and even entire car fires! I will be getting an entire fuse box from the junkyard today, and then purchasing a new HDM relay so that new internals won't cause something to happen down the road for the next year at least. I will also get the pigtails along with the fuse box so that I can transfer over the plastic connectors. I am thinking that this issue with the vehicle only acting up when warm is really because it takes some time for the headlight module... I like to call it a relay but it looks like it is a capacitor type system from the diagram on it... anyways, everything gets warm I am guessing I am having transient voltage's through the plastic and after I replace it and test and everything is good, I will go ahead and accept the answer and hopefully that is it! I am so glad this site exists so that when these type of issues come around and I happen to miss the area where things are happening, I get some help! I've only had to do this twice so far in five years, but it is so different having to rely on yourself instead of being at the dealership with Masters in the shop who you can rely on. And I am happy that in this day and age we have the internet to connect to other experts like yourself! Alright I'm getting ahead of myself I'm just really happy that these things are available to me. I'll update later on tonight.

maybe egr opening too much data stream anything unusual? fuel pressure good?

Customer: replied 1 month ago.
Hey, I'm about halfway done transferring the pins from one connector to the other and I have a used fuse box right here that I'll be installing, another HDM relay as well since it was causing that issue with burning up. Along with other adjacent relays because they could have been affected. But anyways, I pulled the EGR off as one of the first things I did to verify it was completely closed and then disconnected it for testing (reinstalled and disconnected connector). Still had the engine bucking

so does it idle smmoth? what about mass air flow readings? fuel trims let me know

Customer: replied 1 month ago.
I went on a test drive after getting this fuse box installed, everything was fine for 10 minutes. I pulled over and turn the ignition off and restarted and it immediately was acting up. I mean, we are talking completely fine, and then restarting and it was bad. In fact I'm writing this message as I am stranded and it won't restart it just cranks.And now I'm going to do some things with the ignition and I'll make this all one response...Alright I took the key and cycled it from off to position to a bunch of times, and after doing that maybe 10 times, I then finally went to the start position and it started right up. It ran fine for about 10 seconds and then it started cutting out on me. Trying to get back to the shop right now, and I now notice that when it's doing the cut out stuff on me, the RPMs won't drop below about $1,000 but the blower motor is going between fast and slow as it's doing the cut out stuff. I have monitored the voltage last night when I noticed this for the first time and it stays above 13 volts as it is cutting out, so I know that the blower motor isn't slowing down because of a voltage Supply issue, at least not from the alternator. I'm beginning to suspect a bad ignition switch.
Customer: replied 1 month ago.
Alright back in the shop. At idle my short-term fuel trim is too and long-term fuel trim is 0. The mass airflow sensor is .02 lb per second at idle comma and I can get it up to about .08 pounds per second when I brake torque it and the engine starts bucking. The idle is usually always smooth, today it's acting up severely and sometimes that idle dips down a little bit and it will stall if you are doing a lot of testing. But if you leave it alone it's usually okay. And as I'm writing this the idols dipping down and then going back up over a thousand and settling out. As for the mass airflow sensor again it said about 3200 hurts and when I Torque It we can get it up to round 5200. O2 sensors going back and forth between .1 volts and .8 volts even during engine bucking.
Customer: replied 1 month ago.
I'm honoring in the engine bay as it's idling because now all the sudden the idle is becoming erratic. And it is happening with response to the AC cycling on and off.
Customer: replied 1 month ago.
By the way the inside of the ignition switch looked great. All the contacts were squeaky clean.

y​es put proper amount of Freon and that should be ok

ok you need to get a full grade scanner that is able to get u codes if you don't have one

monitor voltage to the coils and pump when it does this no start thing and stall thing....yes maybe wires running everywhere

Customer: replied 1 month ago.
You'll have to excuse me on my absence. Absolutely horrendous personal issues with my ex moving out of state with my son and now I have a giant legal battle to fight.So I went ahead and check out all the wiring and everything seems okay. Since this seems to be a complete mystery issue, I went ahead and replace the lower intake gaskets. I also went ahead and replaced the coolant temp sensor and thermostat because I always do on these jobs. I did find a few things that I usually don't see. One of them was the extreme amount of rust and scale inside of the water jackets. I usually see them with better maintenance. Also somebody used clear silicone RTV to reseal the heater hose bypass pipes, and something else that was very rigged, but I can't exactly remember.The PCV valve under the MAP sensor can be blown through both ways. And I have no idea if that is even what it's operation is supposed to be since it doesn't look like a normal PCV valve. I'll be grabbing one from the store here in about six more hours after I sleep some and see how they are supposed to operate. I'm pretty sure after the are TV sets and I put the new PCV valve in, because I'm just going to buy it, its only $3. I'm sure it will still have the same problem but I thought maybe during the tear down I would see something.

yes I always use oe pcv when possible

Customer: replied 1 month ago.
Alright it still has the same problem however it has a certain smoothness to its Idol and during warm-up there is also better pedal response. But it is because somebody was in there and did a lot of rig jobs putting RTV in certain places like on the heater pipes coming out of the intake, instead of just replacing the pipes themselves or even just putting o rings on them.
I really don't have a reliable way to test those coils because it's running perfect at idle. And the design makes it impossible without special adapters to test the primary firings of the coils so I'm just going to the junkyard tomorrow and pulling off an ignition module and 3 coil packs and swap it all over and see what happens.

k let me know

eric remington and other Pontiac Specialists are ready to help you
Customer: replied 1 month ago.
I pulled some really nice Belden wires off a used car at Pull-A-Part, grab some coils that looked nice and an ignition module and that was it.