I have a 2005 Buick Rendezvous with a 3400 engine. We are the second owners. The first owners had all work done at a Buick dealership. We've owned it for about five years (65k miles). Our first repair about 6 month after we got it was a head gasket replacement. The car overheated and blew the gasket. Repair shop said it was a flaw in the HG design and the new gasket was "beef-ier". A few weeks later we started having problems with it not wanting to go at highway speeds [code P0300). Took it back to the repair shop they replaced plugs and wires and said they could not find anything wrong or recreate problem. A week later we started having the same problem again accompanied with difficult shifting. had a few repair places look at it but could not pinpoint the problem. We did find that if we only used high grade Non-ethanol fuel the power and P0300 did not have any problem, but it still did not shift well. Memorial day weekend 2019 the transmission gave out in Forsyth GA and we spent most of the week there waiting for the local shop to put a new tranny in it. The tranny seemed fine after that but with no non-ethanol gas in the area we started having misfires again. Octane booster and ethanol stabilizer added to fuel helped but still had to baby her on the interstate. the P030? codes only happened with a load at speeds above 60 mph (going up hill on interstate) and now we were also getting code P0101. We learned to keep high grade non-ethanol fuel in the car, not drive on the interstate, and not use cruise control. When we did need to drive on the big road and the problems popped up we could simply clear the codes (while driving ) and everything smoothed out. About a year ago I was pulling a trencher up our Tennessee Mountain (6% grade) and blew the transmission seal. The local shop replaced the seal and some other worn parts. Last week I was driving to Nashville (on the interstate) with little problems but as soon as I got off the Interstate the car started overheating at the first light. I did not have far to go and babied it there. Let the car sit for about 2 hours and then checked the coolant level it was low by about 12 ounces. I filled it up and started home. Driving the 3/4 miles to the interstate the temp gage would get hot then normal, hot then normal. I figured the thermostat was going out. Once I got on the interstate it drove fine at a normal temp, but as soon as I got off it started overheating again. I stopped and let it cool, the coolant level seemed good. I bought two bags of ice to pack the engine and radiator with and started the 30 mile drive home (I would have to climb over a 1000 feet up the mountain to get home) but on the flat I could only drive about a 1/4 mile before it would overheat. I pulled into a church parking lot and got a ride home and a friend helped me get the car home the next day. The car started and ran fine to drive it up onto the trailer, and then when we got home we unloaded the car about a 1/2 mile from my house and it drove home with no problems.

Thursday a pulled the thermostat (had to pull the intake and throttle body to get to it); put it in a pan of water and at about 180 degrees it started opening and was full opened by 195. I did notice a lot of coolant and residue coating everything under the throttle body and thermostat housing. I figured maybe I had a leek and/or air in the coolant system I got a new thermostat, new throttle body coolant hose, new throttle body gasket, and new bleeder valves. I also had to get new sparkplug wires because I broke the #4 plug wire tring to remove it so I could move the heater pipe to disconnect the throttle body. While I had everything apart I doused the engine with degreaser and cleaned the throttle body and intake with carb cleaner. I put everything back together and filled the coolant system and started to bleed the system. When I tried to start the car it would not start and was very rough turning over. It seemed like I got plug wires mixed up. I have checked and rechecked the plug wires and they are in the correct position. I've pulled wires for #* 2, 4,& 6 (the front three plugs) one at a time with no noticeable difference in how the car starts/runs. Ive, swapped #4 and #6 plug wires with no noticeable difference. I've pulled and checked all three coil packs (primary checks at .3 ohms, secondary checks at 6K ohms) the bottoms of the coils were slightly dirty/slimy/greasy but I cleaned them so they were clean and dry. There are no cracks, discoloration, or deformation on the coils or any electrical connectors I can get to. The back bolt on the coil packs were corroded both on the bolt washer and on the metal ring on the top of the bolt hole. I brightened both up with sandpaper and sprayed with battery corrosion inhibitor.
Nothing that I have done seems to make any difference at all in how the car starts or runs. It starts very roughly (insert your own expletive) with determination and pedal work I can get it to start and run then after about a minute or two the RPMs will kick up to about 3k and it will just sit there and race. I am not getting any codes from my OBDII reader.

What suggestions do y'all have for me?

Without codes how did you know what cylinders to go after? If you have misfires and that reader is not picking it up does it connect to the PCM?

Overheating issue: Probably electric fans. Were they ever running?

Not able to idle after changing a thermostat with no codes: I suspect spark plug wires from what you describe.

Bigger picture: There are some leaps of logic here that have proven detrimental to the overall situation. I suggest you bring those issues first before acting. For example:

- Runs fine on the highway then overheats right after getting off: Electric fans.
- P0300 then will only run fine on certain gasoline: Probably fuel pressure low.

Was it the new transmission that blew the seal?

Thank you both for your input. I will try to answer all questions.

Without specific P03?? codes I do not know what plug/wire/coil to go after that is why I checked all three coils and replaced the three wires on the front of the engine (#* 2, 4, & 6) when I only broke the #4 wire. (I did not change the three on the back of the engine because they are difficult to get to and I had not done anything with them).

I've tried two different code readers and they both give me all the live data they normally would so I do believe they are connecting to the PCM (the one even confirms that is is connected to the PCM). When I do get the car to start after about two minutes of extreme rough idling (and a lot of peddle work on my part); the engine will rev up to abot 3,000 RPMs and smooth out. If I let it run at that RPM for a few minutes it will gradually increase to abt 3,200 RPM (at which point I shut it off) and I do get the P0300 and P0101 codes.

Since the car was starting and running before I changed the Thermostat I assumed that the rough starting issue is related to something I did while trying to change the Thermostat (like breaking the #4 wire, getting some of the front three wires crossed, or dousing the engine with degreaser and getting junk into the coils or one of the electrical connectors). I understand what ***-U-ME does so with your suggestions I will start a more systematic approach to resolve this. Starting with Checking all 6 plugs and wires and actually see if I'm getting spark from the coil packs.

I never even thought to check the electric fans (never had one fail before), I will do that once I get the car to start and bleed the coolant system.

I also never thought to check the fuel pressure in relation to the P0300 issue (apparently neither did any of the repair shops I brought the car to) I will definitely look into that (I could save a lot of money buying regular gas instead of High grade non-ethanol).

It was the replacement transmission that blew the seal. It had been replaced about 14 months earlier and had about 16,000 miles on it so it was out of the warranty period. Also I was pulling a very heavy load up a long steep mountain road. That happened about 12 months ago and the car has driven about 10,000 miles since with no transmission issues and only the occasional P0300 and P0101 codes.

check the cooling fans relays , I had the same problem , they would work intermittently. I just replaced all of them . As for the fuel , there is wires that run by the driver door to behide the driver seat to a connector and if you look under the car you will see the fuel pump wires connected to that same connector . I took the connector out , because my wires were corroded. I ended up soldering one wire at a time spliced into from the consul running alongside the drivers door through the floor connected directly to the fuel pump. Pull out the connector from under the car and check to see if any corrosion is in the connector plug or/ and check the wires to make sure they are still good . 2007 rendezvous owner.

Well I pulled all the old plug wires and checked for resistance. All but the new wire on #4 and a different wire on #5 had a resistance of about 5-6 K per foot. The number #5 wire (that looked the same as the wires in the new set I got had a measured resistance of about 500 OHMs per foot. the new wire on the #4 cylinder measured infinite resistance (no continuity at all). I then checked the whole new set and only 2 of the new wires had any continuity each measuring about 2-3K per foot. I replaced the bad wire with one of the good ones and the car started right up but was immediately racing at about 3,000 RPMs, After a few munities I checked codes and there was just a P0101 (MAF out of range code) I cleared the code and the engine immediately reved down and stalled. When I tried to start it again it is having the same rough starting/idling issue. I replaced all six plug wires with a new set from NAPA, I measured each wire and they each had a measured resistance of about 1,000 OHMs per foot. Once I got them all back on and everything together I checked to make sure I was getting spark from each terminal of each coil pack. I am getting good spark. I had my wife start the car while I took the attached video

The beeping on the video is from a booster battery pack that I had hooked up to the car. You can see the throttle cable moving as my wife "worked" the pedal to keep the car running. About 46 seconds in, the engine started to rev up and smooth out so I told her to let the pedal go at which point the car stalled.


Is there anything else I can/should check on the ignition system?
Could this be a fuel and/or MAF sensor problem.

I'd be looking at scan data and see what is going on. Sounds like it could be out of time.

This is how yours is wired, right?

Did you spray the MAF sensor with carb cleaner?

Yes, that is how my plug wires are hooked up.
And yes, I did spray the MAF with carb cleaner.

I just checked the fuel pressure. With the Key On Engine Off (KOEO) I get an initial reading of 50 PSI then after a few seconds it drops to 48 PSI. With the Key On Engine Running (KOER) the pressure drops to 40 PSI, When I turn the key off the pressure rose to 42 PSI.

From what I've found the KOEO reading should be 52-59 PSI and with KOER the pressure should drop 3-10 PSI (that would make the reading in the range of 42-56 PSI).
Is being 4 PSI under the KOEO range and 2 PSI under the KOER range going to give me the problems I am seeing?

I also noticed with this test while watching the live data from my code reader while trying to keep the car idling and "working" the pedal the RPMs ranged from 100 to 1500, and the Timing advance was all over the place. Once the engine reved up to the 3K plus mark the Timing Advanced settled at 40%. I let the car run for about three minutes at 3K plus RPM and checked the codes there were no logged codes bu I did have pending codes or P0300 (random/multiple misfires) and P0101 (MAF sensor out of range).