Hi Everyone!
This is a question about our 1996 Mercury Mystique LS, 2.5L Duratec, MTX, with about 60K original (verified) miles on the odometer. I’m posting this with my husband, since I’m the writer and he’s the mechanic! He’s reached a turning point and is hoping for advice.
Over the past months – and given I only put about 2500 miles a year on the car – the idle began showing an intermittent slight stumble. In the past few weeks, however, when the car is warm (my husband is calling this a hot soak…) it has an intermittent hard start, i.e., the idle stumbles, drops, and may die if the throttle isn’t engaged to rescue it. Generally it starts fine, and otherwise runs great. (My husband says it’s more like a hot soak problem that may indicate a dirty manifold.)
He’s been trouble-shooting a lean condition, and at first discovered a severely damaged PCV hose, which he repaired. Following this repair, the idle had the same stumble problem, and scan data still shows lean conditions at both banks, as follows: Long-term Fuel Trim (LTFT) Bank 1 at 15 points positive; LTFT Bank 2 at 20 points positive. Also, Short-term Fuel Trim (STFT) at each bank lingers around 0 (maybe 1 or two points positive).
Revving the engine to about 2500 RPM brings the LTFT down, at each bank, to close to 10 points positive, or even less, and the STFT stays pretty much the same. This seems to indicate a vacuum leak somewhere after the MAF. The MAF, IAC, EGR, and TPS have all responded properly to RPM changes (so seem to be doing what they’re supposed to), and the throttle plate and IAC have been cleaned and both gaskets replaced (and seem to be working fine).
At idle, he tested the most suspect hose connections and gasket areas with propane, and got no indication of changes in RPM or LTFT. Later on, also at idle, he injected propane under the manifold cover (between the engine block and the manifold area), at the Bank 2 location, and noticed a change in RPM (it bogged down) and also the STFT dramatically dropped to minus 20 points. Therefore, he concluded the propane itself seemed to enrich the conditions, indicating a lean condition where the O2 sensors still seem to managing the fuel trim values around desirable levels (i.e., at zero plus one or two points). Based on this, he concluded that both upstream O2 sensors are working (since they managed the STFT for both banks in this test).
He has also thinks, though isn’t sure of this diagnosis, that either the top or bottom manifold gasket has a slight vacuum leak. However, especially since the computer is able to keep the STFT around 0%, he’s wondering if we should venture into a UIM/LIM cleaning/rebuild project at this point.
Again, apart from the intermittent stumbling idle at start-up when warm, the car runs flawlessly under any load, and though he hasn’t confirmed it, he presumes this means the fuel injectors and fuel pump are okay. Also, there are currently no codes; after the damaged PVC hose was fixed, my husband erased the lean condition code and it hasn’t returned.). We got the car about 8K miles/three years ago, with no maintenance records, so don’t know what’s been done to date. We have not changed plugs or wires, which would otherwise be due.
So, we’re wondering if we should venture into a UIM/LIM cleaning/rebuild at this point, since clearly the idle stumbling issue is getting worse, and we’re worried that leaving it to the computer to correct an apparently lean condition could eventuate in damage to the front catalytic converters – not a cost-effective option.
Any advice will be appreciated!
Thanks.
This is a question about our 1996 Mercury Mystique LS, 2.5L Duratec, MTX, with about 60K original (verified) miles on the odometer. I’m posting this with my husband, since I’m the writer and he’s the mechanic! He’s reached a turning point and is hoping for advice.
Over the past months – and given I only put about 2500 miles a year on the car – the idle began showing an intermittent slight stumble. In the past few weeks, however, when the car is warm (my husband is calling this a hot soak…) it has an intermittent hard start, i.e., the idle stumbles, drops, and may die if the throttle isn’t engaged to rescue it. Generally it starts fine, and otherwise runs great. (My husband says it’s more like a hot soak problem that may indicate a dirty manifold.)
He’s been trouble-shooting a lean condition, and at first discovered a severely damaged PCV hose, which he repaired. Following this repair, the idle had the same stumble problem, and scan data still shows lean conditions at both banks, as follows: Long-term Fuel Trim (LTFT) Bank 1 at 15 points positive; LTFT Bank 2 at 20 points positive. Also, Short-term Fuel Trim (STFT) at each bank lingers around 0 (maybe 1 or two points positive).
Revving the engine to about 2500 RPM brings the LTFT down, at each bank, to close to 10 points positive, or even less, and the STFT stays pretty much the same. This seems to indicate a vacuum leak somewhere after the MAF. The MAF, IAC, EGR, and TPS have all responded properly to RPM changes (so seem to be doing what they’re supposed to), and the throttle plate and IAC have been cleaned and both gaskets replaced (and seem to be working fine).
At idle, he tested the most suspect hose connections and gasket areas with propane, and got no indication of changes in RPM or LTFT. Later on, also at idle, he injected propane under the manifold cover (between the engine block and the manifold area), at the Bank 2 location, and noticed a change in RPM (it bogged down) and also the STFT dramatically dropped to minus 20 points. Therefore, he concluded the propane itself seemed to enrich the conditions, indicating a lean condition where the O2 sensors still seem to managing the fuel trim values around desirable levels (i.e., at zero plus one or two points). Based on this, he concluded that both upstream O2 sensors are working (since they managed the STFT for both banks in this test).
He has also thinks, though isn’t sure of this diagnosis, that either the top or bottom manifold gasket has a slight vacuum leak. However, especially since the computer is able to keep the STFT around 0%, he’s wondering if we should venture into a UIM/LIM cleaning/rebuild project at this point.
Again, apart from the intermittent stumbling idle at start-up when warm, the car runs flawlessly under any load, and though he hasn’t confirmed it, he presumes this means the fuel injectors and fuel pump are okay. Also, there are currently no codes; after the damaged PVC hose was fixed, my husband erased the lean condition code and it hasn’t returned.). We got the car about 8K miles/three years ago, with no maintenance records, so don’t know what’s been done to date. We have not changed plugs or wires, which would otherwise be due.
So, we’re wondering if we should venture into a UIM/LIM cleaning/rebuild at this point, since clearly the idle stumbling issue is getting worse, and we’re worried that leaving it to the computer to correct an apparently lean condition could eventuate in damage to the front catalytic converters – not a cost-effective option.
Any advice will be appreciated!
Thanks.