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Hi,

Just joined but have been using these forums for a while for information. 

SO I have an 88 YJ with the TBI. My issue all started when I went to get it smogged last year. Short story it failed 3 times for high NOX @ 35mph. By the third failure I had replaced every sensor I could replace, including the EGR. The cataliytic converter only has about 1000 miles. I do have the snap-on diagnostic tester, just looking for some direction to narrow my troubleshooting. Currently the jeep wont idle on its own and I can smell that it is running very rich and it seems to run a bit rough.

Thanks!

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JP88YJ...The rich and rough idle are a clue.  NOx is handled by the EGR under normal conditions.   Sounds like your EGR valve vacuum is not applying correctly and/or you have an EVAP canister issue.  This could be a vacuum hose routing problem, a defective/missing EGR Solenoid or no current to the EGR Solenoid at an idle. 

Assuming that the pressure regulator is holding TBI pressure normally at the TBI unit and the fuel return from the TBI is not restricted, check for a vacuum routing issue or defective EVAP canister.  (See our many forum discussions about the regulator pressure, injector leaking, fuel supply and fuel return troubleshooting.) 

Your EGR is controlled by the EGR Solenoid.  It should not get direct vacuum from the intake manifold, rather the manifold vacuum routes through the EGR Solenoid.  If the EGR valve is open at an idle, you should be able to disconnect the vacuum supply hose at the EGR and see a difference in the idle.  If the idle stabilizes and what seems like a rich condition disappears, the EGR valve is open when it should be closed. 

I have attached two PDF vacuum hose schematics for your YJ Model 81 2.5L TBI engine.  Note the hose routing.  The diagrams are similar, the upper PDF also shows a 4.0L for comparison.  Note the 2.5L details, including the EVAP canister Tee and other vacuum features:

Jeep YJ Wrangler 2.5L TBI Engine Vacuum Circuit.pdf 

YJ Four-Cylinder TBI Vacuum Schematic20200406_12500982.pdf

Confirm vacuum routing through the EGR Solenoid.  With the engine idling, check the vacuum at the supply side (from manifold) of the solenoid.  This should be 15 in.hg. or higher.  Now check the vacuum at the EGR side of the solenoid.  If the engine is idling, that reading should be zero. 

Disconnect the EGR Solenoid wire connector:  The vacuum should read the same on both sides of the solenoid.  The EGR valve solenoid is closed by electrical current. Disconnecting the electrical supply will allow the solenoid to open.  If the EGR Solenoid is open at an engine idle, either the solenoid is defective or there is no current to the solenoid.  Check for supply current at the EGR Solenoid wire connector with the engine idling.

 If the solenoid vacuum has been bypassed (vacuum going from the manifold directly to the EGR), your EGR is open at an idle and low speeds.  That will cause a rough idle.

Also check vacuum routing to the EVAP canister.  Check the EVAP function.  Check the fuel filler EVAP system and the gas cap seal.  Also, your YJ 2.5L TBI engine does use a PCV valve as part of the crankcase ventilation system.  Make sure the PCV Valve is in good condition with vacuum applying correctly.  The valve closes at an idle and opens as engine speed increases.  A defective PCV valve will cause a rough idle.  The PCV valve is not expensive to replace periodically.

Moses

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  • Moses Ludel changed the title to 2.5L TBI Jeep YJ Wrangler Running Rich and Won't Idle
  • 8 months later...

Just wanted to give an update on my issue. Found a couple issues that I was able to resolve. The EGR solenoid was not functioning found the ground was bad. fixed that and now it works. I also discovered, after checking my fuel pressure, that I didnt adjust it after I put a new regulator in and it was too high.  I still have the issue with too high of NOX during the 25mph test, Not sure where to go now. Could it be excess carbon buildup?

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JP88YJ...NOx is usually a high upper cylinder temperature issue.  The purpose of the EGR system is to reduce combustion temps below 2500 degrees F, which reduces NOx.  Make sure the EGR is functioning properly and actually recirculating exhaust at the engine speed you describe.  The EGR valve could be sticking closed or clogged.

Another item would be spark timing.  Make sure the base timing is correct, even though timing is actually a function of the ECU.  Use a timing light to verify timing at an idle and also at the 25 mph engine speed.  Make sure the spark timing is not too far advanced.  Avoid the fan when increasing engine speed and checking the timing.

I would also check for an exhaust restriction (muffler, pipes, cat, etc.) or a clogged catalytic converter.  Make sure the oxygen sensor is working properly as well.  If NOx is the only measurement out of specification, my primary focus would be a non-operative EGR or exhaust restriction.

If you do suspect excess carbon buildup, do a compression check.  Higher than normal compression readings would indicate high carbon buildup.  In any case, Sea Foam would do wonders here.  I do a Sea Foam injector cleaning at routine intervals.  Here's my MPI/EFI approach.  You can do a similar approach with TBI.  Sea Foam also makes a nozzle spray cleaner that you can simply spray down the TBI unit's throat.  Follow directions for carbon cleaning:

https://www.4wdmechanix.com/curing-an-engine-knock-with-surr-tools-and-sea-foam/

I do a walk-through of Sea Foam products in this video, beginning at 6:47-minutes.  You'll see the spray cleaner:

https://www.4wdmechanix.com/curing-an-engine-knock-with-surr-tools-and-sea-foam/

Let us know what you discover...

Moses 

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