sbs10v8

89 renix stock except for 703 injectors. My jeep has been running rich, poor gas mileage, erratic idle and some light smoke out the exhaust. After a ton of reading and testing sensors the MAP sensor seems to be the culprit. Im getting good vacuum to the sensor, key on engine off im getting about 5 volts, engine running fully warmed up its dropping to about 3.7 volts. According to everything ive read it should be dropping to about 1.5-2.1 volts. I switched the map sensor with another one I had off a parts jeep and it is exactly the same. Yesterday I replaced the battery ground cable and cleaned all the grounds at the dipstick tube. What else could cause the map sensor not to drop the appropriate voltage?
thanks

cruiser54

Try this test:

Cruiser’s Renix Sensor Ground Test
　

This sensor ground circuit affects the CTS, TPS, IAT, MAP, ECU and diagnostic connector grounds. It’s very important and not something to overlook in diagnosing your Renix Jeep as it is common for the harnesses to have poor crimps causing poor grounds. If any or all of the sensors do not have a good ground, the signal the ECU receives from these sensors is inaccurate.

Set your meter to measure Ohms. Be sure the key is in the OFF position. Using the positive (red) lead of your ohmmeter, probe the B terminal of the flat 3 wire connector of the TPS . The letters are embossed on the connector itself.

Touch the black lead of your meter to the negative battery post. Wiggle the wiring harness where it runs parallel to the valve cover and also near the MAP sensor mounted on the firewall. If you have an 87 or 88 with the C101 connector mounted on the firewall above the brake booster, wiggle it, too.

You want to see as close to 0 ohms of resistance as possible. And when wiggling the harnesses/connectors the resistance value should stay low. If there is a variance in the values when wiggling the wires, you have a poor crimp/connection in the wiring harness or a poor ground at the engine dipstick tube stud. On 87 and 88 models, you could have a poor connection at the C101 connector as well.

Revised 06/12/2012

sbs10v8

I did this test yesterday while troubleshooting and the resistance remained constant, just a little above zero.

cruiser54

Good. The vacuum line from the throttle body to MAP is in good shape?

sbs10v8

Yes...even tried running a different line from a different vacuum port and it did not change anything

Domaas

I had the same problem recently,I had a brand new bad map sensor,found it by back probing the map sensor,then using a vac pump to suck it down and test the sweep.

Tested bad,swapped with a better grade part.

cruiser54

Pins in the MAP harness connector not retracted? Have you tried squeezing them a bit?

sbs10v8

Pins on the connector look good. I put a hand vacuum pump on the sensor while its running and if I manually put about 15 pounds of vacuum on the map sensor the voltage drops to the acceptable range and the rpms increase. I have put a new vacuum line on it, tried different vacuum ports on the manifold, swapped the map sensors out again and it seems like it does not have enough vacuum going to it. I've also checked the entire manifold for vacuum leaks and everything looks fine, the vacuum ball is also connected and looks good if that matters. Kinda lost on what else to try to get more vacuum to it.

sbs10v8

Ok I have pinned it down to the engine only having 5 in hg of vacuum. I've sprayed the entire intake manifold with brake cleaner and I am unable to find a vacuum leak. What else would cause that little vacuum at idle?

Domaas

I would run a compression check, and also check for timing chain wear,slop?

cruiser54

Use this test with your vacuum gauge.


http://www.classictruckshop.com/club...ts/vac/uum.htm

sbs10v8

The needle stays steady just above 5in hg, which makes me think there's a vacuum leak somewhere. Will the engine run if I block off all of the vacuum ports on the manifold then reattach them one by one and try to track down the leak? I'm hoping its a vacuum line somewhere and not something more serious like a cracked head or bent valve. I'll do a compression test tomorrow and report back with my findings.

cruiser54

Why not pinch the vacuum lines off with pliers for diagnosis?

DFlintstone

EGR valve? & x2...pinch, pinch pinch.

highmileage

You could check for plugged vac ports, is your intake clean?