Jump to content
Free Membership Now Includes Posting and Reply Privileges to Help Meet Your Transportation Needs During the COVID-19 Health Crisis! ×

Jeep Wrangler YJ 2.5 TBI : Low Stable Speed Issue

Recommended Posts

Dear Moses

First of all , a lot of compliments for your forum , where i found many useful info

I am a recent italian owner of a wrangler YJ 2.5 TBI . The car has 185000 miles and run pretty fine , except for a irregular ride at low stable throttle opening : i.e. mantaining the engine stationary at 1500 or 2000 rpm (in 1st or 2th gear ) the car jerks and hiccups ;  if accelerating and in wide open condition the engine response and power became ok.

This happens in both cold or warm engine .

The jeep is equipped with an alternative fueling system by Liquefied Petroleum Gas ( rather  common in italy for saving reasons) , switching to LPG fuel the car run perfectly on all throttle conditions , and considering that LPG require a perfect ignition system , i exclude  any spark system issue.

The jeep is without O2 sensor and catalystic converter ( as far as i know this is a standard for '90 export in italy )

A list of checks done:

- vacuum hoses \ PVC \ air intake sys \ EGR & canister solenoid \ wide open throttle switch

- at idle or full open throttle all is ok , so i exclude fuel line , pump or filter issues .

- injector spray patters seems good

- no leakage or unrestrictions appears at exhaust or intake manifolds

Can You kindly suggest where focus troubleshooting and address additional tests ? ( may be fuel pressure or TPS sensor ??? )

thanks in advance




Share this post

Link to post
Share on other sites

Luca, welcome to the forums...As a matter of routine, I would check the fuel pressure and the TPS voltage values.  Set both to factory specifications.  There is much information here at the forums and also at the magazine (www.4WDmechanix.com) on performing each of these tests and the correct values.


I can tell that you are very thorough in your troubleshooting.  However, I have some doubts about the oxygen sensor delete, as the TBI system is dependent on O2 readings to determine the air fuel ratio (A/F) and mixture adjustments.  Do more research here, and if you cannot find an answer, I will look further into the Mopar parts for export models.  I believe you need an O2 sensor signal.  The catalytic converter is a passive, downstream device and may not have been required.


I would also check for vacuum leaks, although your LPG system would like have turned up any leaks by now.  Check the intake manifold to cylinder head seal for seepage. 


A concern with your engine's symptoms is the idle actuator.  This is the equivalent of an idle air control valve.  If there is either damage or a sticky actuator, you would have the idle roughness and surging that you describe.  To help you identify the idle actuator and related parts, here is a parts diagram:


Jeep 2.5L TBI Idle Actuator and Other Parts.pdf


See whether the idle actuator is at fault.  This device might not create a problem with LPG.  However, with gasoline operation, there would be trouble.





Share this post

Link to post
Share on other sites

Hi Moses,

thanks for your kindly reply.


A feedback  regarding other checks completed :

- TPS seems ok (output voltage ranging from 0.4  at idle to 4.6  in WOT ).

- Fuel pressure …perfect ….!!! 14.5 PSI

- Vacuum  in air manifold , bad news  : fast vibrations of gauge needle between 14-20 inHg at 800 rpm , steady at higher rpm , opening and closing drop to 2 and raise to 25  à so i suppose a leak in valves guides ( or seats ?) , not surprising for such  high engine mileage ….( I don’t know if the head was never rebuilt)…anyway we are treating about a vintage car and some imperfections must be accepted ..! 


Regarding O2 sensor  i did not find any additional info , but i saw an other 1990 yj without and so i assume it's a standars for export at that time 


A question : do you think that valve leakage can cause the problem in object during low rpm ride ?


Thanks again


Share this post

Link to post
Share on other sites

Luca...The valve problem could be responsible for a number of issues. Manifold vacuum and MAP are important signals, and manifold vacuum determines cylinder pressures. I wrote the tech columns for Guide to Muscle Cars and Popular Hot Rodding years ago, and for high performance tuning, we referred to an internal combustion engine as a "vacuum pump". Without uniform, adequate vacuum per cylinder, performance suffers.

One thing for sure: With a TBI engine, which is like carburetion in many ways, the uneven cylinder pressures will cause considerable trouble and contribute to rough low rpm performance. The O2 sensor is constantly trying to adjust for uneven cylinder pressures and compression. As you hint, bad valve guides would present more of an issue at low speeds. In my experience, "sloppy" valves and guides do seal better as velocity increases. You should be experiencing valve guide oil leakage and blue smoke out the exhaust if the guides are worn much.  Another possibility, if the valves, guides and seats are actually okay, would be carbon buildup on the valve faces and seats.  This would have to be extreme to produce the vacuum loss you describe.

I would run a cylinder leakdown test on each cylinder at its TDC position (top of compression stroke). This is far more accurate than a cylinder compression test, which may not account for the cylinder taper or wear at the top of the piston ring travel. A leakdown test, by comparison, will be run with the piston static, both valves closed, at the highest point of cylinder wear. I discuss leakdown testing at the magazine and also at our General Repair forum.  I share how to make an inexpensive cylinder leak down tester...We can discuss this further.

A simpler compression test might reveal some compression loss. Your manifold vacuum test, with a widely swinging needle, is a classic and time-honored method for pinpointing worn valve guides.


Share this post

Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.


  • Create New...