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Stock XJ Cherokee Tech. All XJ Non-modified/stock questions go hereXJ (84-01)
All OEM related XJ specific tech. Examples, no start, general maintenance or anything that's stock.
Engine: 4.3L with headers and full 3" exhaust system
Originally Posted by BlueRidgeMark
I don't believe it is. A vacuum leak will add air to the air-fuel mix, making it more lean, which will cause the ECM to add fuel. The ECM is monitoring the TPS and the O2 sensor, adjusting the fuel accordingly. The vacuum leak messes up the relationship between the TPS position and the amount of air in the mix. It's like having a mis-calibrated TPS.
Originally Posted by cruiser54
I've never been able to wrap my head around an exhaust system under pressure pulling outside air in......
Not to mention how many Jeeps I've had with cracked exhaust manifolds, and when observing the fuel trims, oxygen sensor movement, and MAP readings with the factory tester, were all fine......
Yes, it is counter intuitive isn't it. But consider basic header operation over a standard cast manifold. We are dealing with a medium length header here. The idea of the tube for each cylinder is to create exhaust gas velocity. As it passes the crack, you are right it is under pressure. However, the effect of this velocity in a race engine with overlapping cam lobes is that once the exhaust goes is empty it will undergo a short period of vacuum due to the momentum of the exhaust gasses. This is used in a race engine so that the exhaust and intake valves are open simultaneously so that the momentum of the exhaust gasses create more vacuum localized in the cylinder and draw in more fuel/air than under normal conditions. It is under this phase of the pulse that air is drawn in a crack and thus fooling the O2 sensor into thinking the mixture is too lean
Engine: 4.3L with headers and full 3" exhaust system
Originally Posted by BlueRidgeMark
I don't believe it is. A vacuum leak will add air to the air-fuel mix, making it more lean, which will cause the ECM to add fuel. The ECM is monitoring the TPS and the O2 sensor, adjusting the fuel accordingly. The vacuum leak messes up the relationship between the TPS position and the amount of air in the mix. It's like having a mis-calibrated TPS.
You might intuitively think so, but consider the "Log" intake manifold on pre 99 4.0s. If a vacuum fitting on the log leaks, that air entering is distributed in the vacuum air that does not have any fuel yet. Most leaks of this nature are compensated for by the IAC with no net effect. If not, it idles too fast. If however the leak is on the runner, say at the head, then some extra air can be drawn into that cylinder and if serious enough it can cause a miss. But remember, that change is spread over the mixture of the other five cylinders. Under throttle, vacuum goes down and air volume through the throttle body goes up so that effect is minimize after averaging all the values
Originally Posted by cruiser54
I've never been able to wrap my head around an exhaust system under pressure pulling outside air in......
Not to mention how many Jeeps I've had with cracked exhaust manifolds, and when observing the fuel trims, oxygen sensor movement, and MAP readings with the factory tester, were all fine......
Yes, it is counter intuitive isn't it. But consider basic header operation over a standard cast manifold. We are dealing with a medium length header here. The idea of the tube for each cylinder is to create exhaust gas velocity. As it passes the crack, you are right it is under pressure. However, the effect of this velocity in a race engine with overlapping cam lobes is that once the exhaust goes is empty it will undergo a short period of vacuum due to the momentum of the exhaust gasses. This is used in a race engine so that the exhaust and intake valves are open simultaneously so that the momentum of the exhaust gasses create more vacuum localized in the cylinder and draw in more fuel/air than under normal conditions. It is under this phase of the pulse that air is drawn in a crack and thus fooling the O2 sensor into thinking the mixture is too lean
Year: 1998 Classic (I'll get it running soon....) and 02 Grand
Model: Cherokee (XJ)
Engine: 4.0
Originally Posted by 4.3L XJ
You might intuitively think so, but consider the "Log" intake manifold on pre 99 4.0s. If a vacuum fitting on the log leaks, that air entering is distributed in the vacuum air that does not have any fuel yet. Most leaks of this nature are compensated for by the IAC with no net effect. If not, it idles too fast.
Not so fast. The IAC can only adjust so much. If the leak is beyond that range, it will lean the mixture. And if you are above idle, the TPS issue kicks in. The air leak essentially throws the calibration off, as there is more air entering the cylinders than the ECM thinks there should be. Again, lean mixture.
Engine: 4.3L with headers and full 3" exhaust system
Not a race engine. Headers only really start increasing performance when the intake and exhaust openings overlap. It is called scavenging. With enough overlap, you get unburned fuel and air coming out the exhaust before the intake stroke really gets going. Hence the crappy idle
Found what could be the source of a sizeable vacuum leak. Mickey mouse repair on a rubber vacuum coupling on the valve cover. I disconnected the vacuum line at the manifold and plugged it. We’ll see how/if things change.
Well it was mentioned there could be a vacuum leak, and Cruiser54 suggested making sure the intake manifold bolts are snug. A tear in the rubber coupler is equivalent to a vacuum leak in the manifold I would think.
03-24-2018, 09:40 AM
