Rough idle and using a lot of gas



1994 buick lesabre (3.8 eng.)with 164000 miles. I had what I determined to be a leak at the exhaust manifold gasket but ended up doing the head, intake, exhaust, upper plenum (note: not egr melting type) gaskets (basic engine gasket kit) The car was burning antifreeze through the leak, so I just did the whole job. Heads were good, everything else looked great etc.

I can’t tell if this was the problem prior to working on the car because I didn’t run it a lot once the problem surfaced. Now, the car idles rough, has low power, and is kid of jerky. On top of that I’ve gone through 1/2 tank of gas in 3 days driving no more than 60 miles I’d guess.

It seems to smooth out after it warms up, but only slightly. I’m planning on replacing spark plugs, wires, and checking vacuum lines etc. Is there anything else that might be worthwhile looking out for?


typo! That should have been 160 miles - not 60


I’m no tech, but since you done all that work, the EGR should be clean, BUT…is it?


Without car in hand, my guesses would be a vacuum leak or possibly an EGR valve sticking partially open. The EGR is essentially a vacuum leak, although it’s supposed to be a controlled one. :slight_smile:

My opinion, as usual, would be to connect a vacuum gauge to an intake manifold port. This will tell you in a second if the problem is vacuum related or not.
Vacuum gauges are cheap and I highly recommend having one.


Well, did some looking around and there don’t seem to be any vacuum leaks. There was a line I found near my fuel tank that had a rubber hose piece missing. It looks like it’s some air line for the fuel tank. In any case, when I’ve started it up and punch the gas the intake manifold makes a sort of puff sound, like it’s cutting out but then revs up, so I’m leaning toward testing the vacuum lines and trying to figure out how to troubleshoot a sensor.
The exhaust does smell a little rich at times, and the exhaust seems to really puff hard coming out the tail pipe.
We’ll see how testing goes and changing plugs/wires…
Any other ideas? Would a computer code scan help? I haven’t seen the check engine light turn on.


[b]Anytime an engine ingests coolant, the O2 sensor(s) are damaged. The ethylene glychol in the anifreeze contaminates the sensor. So replace the O2 sensor.

The next thing to check for is a leaking fuel pressure regulator. The ignition system on your vehicle is a wasted spark ignition system. This means when a coil fires, it fires two cylinders at once. One cylinder has air/fuel and ignites while the other doesn’t and nothing happens in that cylinder.

However, if there’s a leak in the fuel pressure regulator diaphram, fuel can be pushed into the engine via the vacuum hose between the regulator and intake manifold. This fuel can pool inside the intake manifold. Then when you run the engine, this fuel is ignited by one of the cylinders that has the wasted spark. And this is what causes the popping noise in the intake. And a leaking fuel pressure regulator will cause lousy fuel mileage.

Locate the fuel pressure regulator on the fuel rail and remove the vacuum hose from the regulator. If there’s even a hint of fuel at this connection, replace the fuel pressure regulator.



That O2 sensor is probably the first thing I considered, and the last thing I planned to replace due to the cost. Your recommendation sounds great though. Thank you.

I did look at the fuel pressure regulator but did not smell any traces of fuel, but I’ll double check.

Thank you for your time. It means a lot to me.