PontiacBonnevilleClub.com

Hello all again. I was the Bonneville with hard/long start a year ago. To give you some background on the car, my last post was about the timing chain causing my last issue. Well, three weeks after I had the timing chain replaced, the engine blew up. I let it sit until August when I decided to replace the engine. It now has a new re-manufactured engine from the GM Dealer. Now, to my issue. I cannot press the throttle more than half or it shutters, skips, or tries to buck. This is more common above 40 MPH, but has happened at lower speeds. I also cannot hold the accelerator steady or her RPMS will jump a few hundred. I have to take my foot off the accelerator completely to make it stop. When I first start her up, she runs rough for about 30 seconds or less and then clears up. She always starts within the first crank or two. I have had her start, run extremely rough and die after a few seconds. She would not stay running unless I pressed the accelerator to clear her up. I had a strong fuel smell before she finally started. If I do not wait for her to completely warm up, she runs like she has a slight miss like every few seconds a hiccup, but the RPMs are smooth. I would think bad cat, right? Well, I just had it replaced two weeks ago and this issue started the day after the new meow was replaced. It is going back in tomorrow for a motor-vac. The mechanic that installed the engine thinks that carbon build-up is causing the problem. I was not able to get the cat fixed right away. The CEL for the cat was on for two months. If this does not fix the issue, what should I look at? Below is a list of parts that have been installed since I bought the car.

Thermostat
Ambient Air Temp Sensor
Temp Sensor
Mass Air Flow Sensor
MAP Sensor
Evaporative Emissions Vent Valve Solenoid
Cat. Converter
Throttle Position Sensor
Oil Pressure Sensor
Plugs (new plugs installed when engine was installed)
Wires
2 Coils (1 old still on the car)
Fuel Filter
Fuel Pressure Regulator
UIM (the one recommended by this forum...not Dorman)
6 Fuel Injectors & O-Rings
IAC Valve
Crankshaft Position Sensor

Everything was replaced prior to the new engine. There are not many parts left that could affect this. What does not make sense to me is no check engine light. When it was hooked up to the computer at the shop is said it was running lean. Short term fuel trim was at 15 and long term fuel trim was at 16. A/F ratio was at 14.7-1, it was in closed loop and the O2 sensors were reading 35mV for upstream and 13mV for down stream. They were switching like they were supposed to. My fuel pressure was at 47 and holding during idle. I will have them check it again at start-up and idle when it goes into the shop. Any ideas would be great. I do not want anything to happen to this engine, I have only put 4,000 miles on it. I will try to get video of what it is doing.
Thank you in advance!

Mine was doin this n it was a clogged cat. Have u checked to see if it glowing?

The catalytic converter was just replaced two weeks ago. Are there bad cats outta the box? Why is there no check engine light for it now when there was one before?
Thanks,
Aly

hmm anything is possible i suppose. have u checked the old coil and icm? they r known to go bad aswell. those might throw code tho.. mine has a bad maf so it shutter but i have the cel and code. i kno sensors are touchy unless u get delco for this car

No, I have not checked the coils in over a year. When I was having the no start issue that ended up being the timing chain, I had the icm tested. It passed on the NAPA tester. I made them test it 6 times to get it nice and hot. Never failed. My only concern is that there is no CEL. Everything is running within its parameters according to the scan tool the shop uses. I will have them test the coils and icm.

Check for vacuum leaks. Check all the tubing and then spray some carb cleaner around the UIM and hoses to see if the idle changes.

Was going to spray the engine with carb cleaner, but I finally got a check engine light. PO171. It only stayed on for an hour, but it was enough time for me to go to OReilly and get it scanned. It is at the shop now. Hopefully they can figure out what is causing the lean mixture.

They want to replace the fuel pump, fuel filter, and do a motor vac after those two are replaced. What do you think?

I would check the fuel pressure with key on / engine off and then with engine on. Then tap the accelerator and see what it is. Fuel filter is a cheap item to replace if it is old.

Before replacing the fuel pump (can be pricey), I would look for vacuum leaks. Check the tightness of the air intake duct, condition of vacuum lines to the FPR, MAP sensor, brake booster. Spray carb cleaner at each connection and listen for a change in idle. Spray around the UIM and LIM gasket areas and listen for idle to change. Basically looking for anyplace air is getting in after the MAF which measures the air so the PCM knows how much fuel to put in.

They sprayed down the engine to find leaks. Could not find any. Rigged a vacuum line up from the throttle to fpr because of a cracked hose, but it was not leaking enough for the idle to change. They determined the fuel pump, filter, and motor vac should be done. The filter was changed when the new engine was put in. So far she is running better than before. The fuel pressure regulator vacuum line will be in on Thursday from the dealer. I will change out what they rigged up with the new line. Hopefully I will not have any more issues...