98 Malibu runs rough/check engine light

[sickcars]

Hey guys I just did head gaskets on this and after I started it up for the first time I got the check enging light on and flashing and the codes are p0300 (random/multiple cylinder misfires) and p1200 (fuel and air metering problem).
My first thoughts were that I had left a couple connectors loose on the injectors but they are all fine. I removed the upper intake and the all the wires are free and no pinches.
I did notice that the Fuel pressure regulator had fuel leaking from the top at the vacuum connection port is that normal? Is it possible a bad regulator could cause misfires? Also the vacuum line connects to a switch of some sort on the back of the intake/infront of the coil packs. Could it be that this switch is bad and causing my problems?
Anyone ever run into this problem before? What was your fix? I'm just looking for a place to start looking.

Thanks guys!

[rhandwor]

If the fuel pressure regulator had fuel coming out of the vacuum connection it is defective. Replace it and do a recheck.

[sickcars]

Thanks for the tip!

I wanted to post my findings on this issue because I think it could be a common mistake.

The Fuel pressure regulator was defective but that was not the cause for the check engine light.
The upper intake had 2 hair line cracks around the bolts in two seperate areas. The reason for these cracks is unsure but it could be from incorrect bolt tightening sequence or it could be from a warped manifold. Either way these Hair line cracks will cause these DTCs and make it run rough from the vacuum leak. The easiest way to find these cracks is a can of carb cleaner. With the engine running spray the carb cleaner around the bottom of the upper intake and if there is a leak there will be a noticable change in the engine idle. Unfortunetly you will need to buy a new upper intake and the only place I've found that has it is the chevy dealer.

I hope this helps people that are looking at their engines and saying WTF is wrong with you!!


Kev~

[manicmechanix]

Yeah that's a good tip. These upper manifolds have been known to crack. I don't remember the UIM having a specific tightening sequence like the LIM but when removing it's very important to gradually loosen the bolts in a diagonal/circular pattern starting from the ends. And when reinstalling reverse the pattern starting at the center and working diagonally out and gradually tightening to the torque spec.

You might be able to find a good UIM in the junkyard. I imagine GM wants a bundle for what amounts to a slightly machined hunk of aluminum lol.

[J-Ri]

There are tons of 96-99(or 2000? whenever they changed the EGR) upper intake manifolds in yards. I had one break on me once, it was the newer style (the one they don't have). The cause was the shop that did it before had "cleaned" it too well and the right front bolt hole was missing some metal. After cleaning it with a rag it was obvious that they ground a bit of a taper into it. Upon torqing it to 18 ft-lbs, it snapped.