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Horrible Idle Renix

sam24th

NAXJA Forum User
Location
charlotte NC
So lately I have had a bad idle once the engine warmed up. Otherwise the truck drove perfectly fine. Today I thought I might aswell replace the O2 sensor incase that was what might be causing the erratic idle. After having replaced the o2 the truck idles even worse than it did with the old o2. I threw the old O2 back in it and it idles much worse than it did before. Any ideas what it might be? I think it might just be coincidence that the O2 was replaced and my idle worsened. It was getting late and I didnt get a chance to bust out my multimeter. Tomorrow I plan on testing the TPS and checking the IAC to see if its sticking.

Info:
-1990 Renix model
-4.0 Aw4 transmission

symptoms:
-idle stumbles when starting, and while stopped at stoplights and stopsigns. Anywhere between 300-900 RPM erratically.
-acceleration is nolonger smooth and seems to be bucking and backfiring.
-very rich smell in the exhaust

Parts checked:
-sparkplugs
-wires
-cap
-rotor
-New O2
-I changed the MAP out with 3 other ones and all of them did the same thing and had no affect on the truck.
 
1) Pull the vacuum line to the fuel pressure regulator on the front of the fuel injector rail, if there is any gasoline, the FPR diaphragm is leaking and causing the problem, and the good O2 sensor is fighting to deal with a fuel leak into the manifold, and loosing the battle.

2) check for a vacuum leak, hose knocked off while replacing the O2 sensor?

3) TEST the sensor grounds with that meter next!!! See:

http://www.naxja.org/forum/showthread.php?t=1072534&highlight=cruiser54+renix+ground&page=5

http://www.naxja.org/forum/showthread.php?t=1012701&highlight=oxygen+test
 
I know I checked the MAP sensor vacuum line thoroughly today so that shouldnt be an issue. I will check the pressure regulator tomorrow morning. Should I do this while the engine is running or not? Thanks for the help so far guys.
 
You need fuel pressure, and any leak will be more visible with the engine running, so yes do it with the engine running.
 
Ok so tested the tps and adjusted per the instructions on the forum. It decreases steadily with the throttle. I also refreshened all the grounds. It still has the same idle issue. All vacuum lines checked out and I traced all of them to make sure. I will check the fuel pressure next after lunch. The pressure regulator doesn't leak also. This has got me scratching my head. Can the egr be causing these problems?
 
Last edited:
Which TPS instructions did you use?

Check your EGR while you're at it:

TESTING THE EGR VALVE FOR 87-90 4.0 ENGINE
Valve Opening Test
1) With engine at normal operating temperature and at idle,
rapidly open and close throttle. Open throttle sufficiently to obtain
at least 1500 RPM. Movement should be noticed in EGR diaphragm.
2) If diaphragm does not move, probable causes are: faulty
vacuum signal to EGR, defective EGR diaphragm or defective
backpressure sensor diaphragm (if equipped), or leaks in vacuum lines
or connections.
Valve Closing Test
1) With engine at normal operating temperature and at idle,
manually depress EGR valve diaphragm. RPM should immediately drop,
indicating that EGR valve is not leaking and had been properly cutting
off exhaust gas flow at idle.
2) If there is no change in RPM and engine is idling
properly, exhaust gases are not reaching combustion chamber. Check for
plugged passage between EGR valve and intake manifold.
3) If engine idles poorly and RPM is not greatly affected by
manually moving diaphragm up, EGR valve is not closing off exhaust gas
flow. Check for carbon between pintle, leaking EGR valve gasket or bad
EGR valve.
***************
 
Fuel pressure checked out ok. I got 32 psi while running and 39 with the vac line disconnected. I just removed the egr and did a block off plate and it still does it. TPS was adjusted using the reference voltage method. I set it to 83 percent of the reference voltage and ended with 3.82 volts.
 
That method checks the transmission side of the TPS, not the engine side. Therefore you can have the engine side of the TPS bad and not know it cuz you checked the trans side. Use these instructions and be sure to do the sensor ground test in the beginning.

RENIX TPS ADJUSTMENT
Before attempting to adjust your TPS be sure the throttle body has been recently cleaned.
It's especially important that the edges of the throttle butterfly are free of any carbon build-up.
IMPORTANT NOTE: 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.
If you see more than 1 ohm of resistance some modifications to the sensor ground harness will be
necessary. The harness repair must be performed before proceeding.
I can provide an instruction sheet for that if needed.
MANUAL TRANSMISSION:
RENIX manual transmission equipped XJs have a three-wire TPS mounted on the throttle body.
This manual transmission vehicle TPS provides data input to the ECU. The manual transmission
TPS has three wires in the connector and they're clearly embossed with the letters A,B, and C.
Wire "A" is positive.
Wire "B" is ground.
Key ON, measure voltage from "A" positive to "B" ground by back-probing the connectors..
Note the voltage reading--this is your REFERENCE voltage.
Key ON, back-probe the connector at wires "B" and "C". Measure the voltage. This is your
OUTPUT voltage.
Your OUTPUT voltage needs to be seventeen percent of your REFERENCE voltage. For
example: 4.82 volts X .17=.82 volts. Adjust the TPS until you have achieved this percentage. If
you can't achieve the correct output voltage replace the TPS and start over.
AUTOMATIC TRANSMISSION:
RENIX automatic transmission equipped XJs have a TPS with two connectors. There is a flat three-
wire connector, same as the manual transmission vehicles have, and it is tested the same as the
manual transmission equipped vehicles--FOR ENGINE MANAGEMENT RELATED ISSUES.
However, the automatic TPS also has a square four-wire connector clearly embossed with the letters
A,B,C, and D. It only uses three wires and provides information to the Transmission Control Module.
Key ON, measure voltage between "A" positive and "D" ground. Note the voltage. This is your
REFERENCE voltage.
Back-probe the connector at wires "B" and "D". Measure the voltage. This is your OUTPUT
voltage. Your OUTPUT voltage needs to be eighty-three percent of your REFERENCE voltage.
For example 4.8 volts X .83=3.98 volts. Adjust the TPS until you have achieved this percentage.
If you can't, replace the TPS and start over.
So, if you have an automatic equipped XJ your TPS has two sides--one side feeds the ECU, and
the other side feeds the TCU. If you have TRANSMISSION issues check the four-wire
connector side of the TPS. If you have ENGINE issues check the three-wire connector side of
the TPS.
For those with a MANUAL TRANSMISSION--the TPS for the manual transmission XJs is
stupid expensive. You can substitute the automatic transmission TPS which is reasonably priced.
Revised 11-28-2011
 
Wow I finally got the damn thing fixed. I decided to throw in a set of new sparkplugs anyways cause I had some in the garage. Long story short, the sparkplugs that were in it that I changed out maybe 4 months ago was causing it to misfire and idle badly. It idles perfectly now and acceleration is smooth as butter. Thanks for the input though guys. It also seems to idle smoother now with the new O2. Im sure readjusting the TPS and freshening up the grounds probably helped some also.
 
They were the champion coppers. I always used them in the firebird and jeep. I guess one of the ones I put in last time was just bad or went bad. Visually the plugs looked fine though.
 
I wonder if new spark plugs from China are bad now? I have had 2-3 bad ones the last 2 years myself. All single platinum, one Bosch IIRC, 2 autolite, one each on two different vehicles. One was physically damaged, maybe from repeated removal, hard to say. They worked for a while (3-9 months), but the others have lasted up to 5-6 years.
 
It may be the case, cause when I first had my YJ I remembered the plugs going out on it shortly after and the original owner had just replaced them with autolites. Then when I bought my XJ I replaced the plugs when I bought it with the champion coppers. A year later one or 2 of the plugs went out and I replaced them again. Now Im replacing them again....
 
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