Sensor/ECM issues with stalling/black exhaust

Sensor/ECM issues with stalling/black exhaust
I have a 1990 XJ, 4.0 Renix engine. This last summer after going through deep mud up to the rear windows it started running rough and having trouble stalling once it warmed up. It would stay running at high speeds but not trail speeds. We replaced the TPS yesterday. It didn't change anything. They gave us the wrong Coolant Temperature Sensor yesterday and got the right one today(the old one was definately bad). After changing it, it ran fine for a while then suddenly ran extremely rich with black exhaust and died then would not restart until it cooled down. We changed the rotor, cap, wires, and plugs along with the fuel filter while it cooled down since these needed to be done anyways. It continued to run very rich and die. We traced down several melted wires to the Coolant Temperature Sensor, O2 Sensor, and Knock Sensor. This may be why it suddenly died (from movement & shorting out) but the wires had been melted for a while. After fixing the wiring it continued to run very rich with black exhaust and die and now it won't even start after the initial second or two of running. We think it is either the O2 sensor or the ECM. We'll try that later. There does not appear to be any vacuum issues and the MAP sensor seems to be working.

Anyone have any other ideas?

You were right to suspect the coolant temp sensor as that is directly related to fuel air mixture.

If you're confident that circuit is now working properly, then I would move next to the MAP sensor, again critical in determining mixture.

1. You mention that you think the MAP sensor seems to be working. Did you test it? That is the only conclusive way to know. A meter and a manual will get you there.

2. Testing fuel pressure with a gauge at the fuel rail is never a bad idea in case you have a fuel delivery problem (ruptured regulator, etc). If memory serves me correctly, I think you want around 31 psi at idle. Someone correct me if I'm wrong on this spec.

3. If fuel pressure and the MAP checks out, it might be valuable to get a scan tool on this to see if you can get some real time data to point you in the right direction.

If you were in mud up to the back window the O2 sensors were submerged. The O2 sensor has to compare oxygen in the external atmosphere with oxygen left in the exhaust gasses. You probably plugged up the vent in the sensor, shorted it or otherwise fubared it.

Try unplugging the O2 sensors so the ECM doesn't get any signal from them and see what it does.

sjkimmel99 said:

If you were in mud up to the back window the O2 sensors were submerged. Try unplugging the O2 sensors so the ECM doesn't get any signal from them and see what it does.

Click to expand...

Excellent idea. Especially if these symptoms started IMMEDIATELY after submerging it.

That is also why I recommended a scan tool. Real time data as there is no check engine light on the Renix engines.

Hopefully I'll get a chance to try these ideas Monday.

Sounds like a sensor related problem when the injection goes into closed loop operation. I've seen EGR problems, bad O2's (BTW was the harness misrouted by the exhaust manifold instead of over the power steering pump?), and a plugged vauum line to the MAP sensor cause this type of thing. I'd look for water/crap in connectors based on your burying it in the mud.

Everything you ever wanted to know about Renix sensors:

http://www.lunghd.com/Tech_Articles/Engine/Basic_Sensors_Diagnostics.htm

You also need to go through the wiring harness, every connector to a sensor and all the other harness connectors, pull them apart, clean them, and then use dielectric grease.

XJs are not submarines.

joe_peters said:

Everything you ever wanted to know about Renix sensors:

http://www.lunghd.com/Tech_Articles/Engine/Basic_Sensors_Diagnostics.htm

Click to expand...

This is the same link that I have been using. It definately is very informative.

We tested the MAP and it definately is good. Now we need to test the fuel pressure.

joe_peters said:

Everything you ever wanted to know about Renix sensors:

http://www.lunghd.com/Tech_Articles/Engine/Basic_Sensors_Diagnostics.htm

You also need to go through the wiring harness, every connector to a sensor and all the other harness connectors, pull them apart, clean them, and then use dielectric grease.

XJs are not submarines.

Click to expand...

this is also good to have

http://www.lunghd.com/Downloads_and_Links/Jeep_Renix_Fuel_Injection_manual.pdf

We tested the fuel pressure. It was consistently 40psi. I found an ECM from a 1989 XJ. It was a little different with two ports rather than just one. We tried it and it would start but not continue to run. I need the correct ECM. Does anyone have an extra ECM?

You have a problem with the fuel system.

Correct pressure is 31 psi with vacuum applied to the fuel pressure regulator, then 39 psi with vacuum to the fuel pressure regulator removed.

If you are getting constant 40 psi check the vacuum to the regulator, if you have vacuum replace the regulator.

It was the ECM. The TCM is behind the dash on the passenger side and the ECM is on the driver side. Soon as we switched the ECM it ran great. The larger plug on the ECM is the same as the one on the TCM.

Have you tested the CKP? The connector for it is at rear of head, driver sie.. is a 3-pin connector with 2 wires.

ECM on the left and TCM on the right.