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I’m working on my neighbors 2001 Buick century 3.1l that has a stumble problem. First some history the car has 105,000 miles and I just replaced the leaking lower intake and timing cover gaskets and replaced the sparkplugs, wires, fuel filter, thermostat, while I was there. The problem she has is at around 25-35 mph the car will stumble and fall on its face but if you feather it until about 50 mph then its ok. It was doing this before everything was replaced and I was hoping me fixing all the coolant leaks and doing the tune up would fix it but no luck. I messed with it for a while and found out if I unplugged the EGR it will run fine with no stumble. So I got a new valve but with the new valve it does the same thing as the old valve, so I think my problem is elsewhere. Also the EGR passages are clean. I’m thinking the converter is semi clogged and that’s causing excessive backpressure and when the EGR opens it stumbles. Any Thoughts
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as you can see from my avatar, my first car was a 2000 Buick Century custom, good car… i loved that car, ill help you the best i can.
the first thing i would do is run a fuel system pressure test, the fuel pumps in these cars are known for going out, sometimes doing funny things, i think fuel pressure should be somewhere around 45-50PSI… cannot exactly remember, but if fuel pressure is low its going too act funny unless you feather the throttle like you are doing, i think that may richen the mixture and may be helping your shutter problem along.
check the fuel regulator, sometimes the fuel regulators go bad in these cars, its a easy as peasy fix just depressurize the fuel system first! i learned the hard way when i did her fuel lines too depressurize the fuel system when doing fuel system work, i took a shower in gasoline while she was on a lift and i was cutting a fuel line too her old rusty fuel filter! if the inside of the fuel regulator smells like gas, replace…
run some Lucas full fuel system cleaner through the fuel system, maybe its a little clogged up or dirty or fuel injectors fouled, seafoam even, there is this brand i am trying on my regal that works good too, not sure what the brand name is… sometimes that helps a little
check your plugs and wires, these engines aren’t super finacky but still… i installed NGK V Powers in my Century and it ran great, these cars like NGK and most of all AC Delco plugs.
check for any vacuum leaks, i remember one time driving her she started too buck and kick and the engine light came on, the hose too the PCV came loose from the intake plumbing and that caused some awful results… there is a little plastic tube that runs from the rocker cover under the ignition coils… too the intake tube, sometimes that pops loose, and i have noticed that can cause some drivability concerns.
hope this helps
OH
with a catalitic converter clogged typically drivability concerns are noticed the heavier engine loads get…
ALSO
the MAF sensor, inspect it see if there is any damage too the little sensors, spray some MAF seonsor cleaner on it liberally, and then let it set for a little while too dry and then put it back in… see if that helps.
When I changed the plugs and wires I used acdelco, I’ve also cleaned the maf. Right now shes driving it with the egr unplugged and it runs fine no stumble but if I plug it in the car stumbles when the egr starts to open at about 25-35 mph and that’s why I have to feather it. But once I get up to cruising speed about 50 mph it goes away so that’s what makes me thing its the cat and excessive backpressure choking the motor when the egr opens. On these 3.1l the egr has a 3/4 tube that runs from the valve into the exhaust manifold and if the cats clogged that’s a lot of volume.
No check engine light?
http://easyautodiagnostics.com/gm_egr_test/gm_egr_valve_1.php
The check engine lights on but only because I have the valve unplugged before that it never threw a light.
You need a scan tool to read the egr position data. I had one that was whacking the egr to 100% on light throttle and it turned out being a bad PCM.
I got a backpressure tester that screws into the o2 sensor hole today and tested the backpressure. The spec acording to alldata and mitchell is it should be less the 1 1/4 psi and anything over 2 psi is clogged. I was right at 1 1/4 psi at idle and about 3-4 at 2500 rpm so it looks like the converter is partiallty clogged. I will update what happens next week when I replace the converter.
Replaced the converter today, cleared the egr codes and drove it everything’s fine. The old converter was plugged and weighed about 20 pounds.
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