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Hi, 

     I've been hunting and chasing my tail to get some kind of throttle response from my wrangler above 45 MPH. I know these are not very powerful to start but I feel like what I'm experiencing is abnormal. Anywhere above about 45MPH when throttle is given There is little or no response from the engine. I have gone through the tuning and troubleshooting, replaced numerous things and adjusted what is able to be adjusted to the best of my understanding. Most recently I put a new cylinder head on due to a cracked web between valves on cylinder 4. I was so sure this was going to solve my issue considering the cylinder was down to 70psi compression. Though the engine runs smoother and quieter now it still is lagging at higher speed. 5th gear is pretty much useless unless going down hill. I have access to a modis scan tool so I can see ecu data. What is bothering me at this time is the engine rpm's are reading about 400 rpm's lower than my tach. Idling around 575 rpm's according to the scan tool data but reading around 1000 on the tach. In my opinion it feels like it is closer to the 1000 rpm range but I don't have a way to check it at this moment. My question is where does the ECU get the engine speed? is it from the ICM or the CPS? If its the cps can the cps fail in a way that it still reads but reads low? I may be barking up the wrong tree and I likely am. but right now I'm trying anything i can think of. Any other suspected areas to check? Any scan tool data that would be more important than others? Any suggestion is welcome. 

Thanks

Dan

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wondernad...Some basic questions before plunging.  You offered a lot of information, but I'd like to know more about the YJ.  Do you have modifications like oversized tires, axle gearing changes, or aftermarket engine changes?  What are the axle ratios—if you know?  Sluggishness can have a variety of causes.  Some are engine tune or ignition/fuel issues.  Other times, there can be problems related to gearing or speed sensors.  You hint about tach error, this could point to parts mix-ups and other concerns. 

Further insights, please...

Moses

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The tires are 31's. The engine is bored 20 over. Everything else is stock to my knowledge. I haven't changed gears and am assuming the previous owners haven't either. I have checked tps adjustment, fuel pressure(14 psi), WOT adjustment and ISA motor is adjusted. All of the sensors react to throttle input when looking at the scan data. 

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wondernad...You've been very thorough and have access to the many 2.5L TBI troubleshooting exchanges here at the forums.  No point rehashing those facts, data and tips.  Sounds like you worked through many of the common problems, research here is helpful. 

The more "original" your YJ is, the easier it will be to diagnose troubles.  There are basic issues related to sluggish performance that I would pursue:  1) fuel supply volume, 2) base ignition timing, 3) manifold vacuum, 4) valve timing and 5) base ignition timing.  Of course, any air flow restriction (air filter, etc.) on the intake side could cause sluggishness.

You can have adequate fuel pressure at the TBI regulator and still not have an adequate fuel supply volume.  Confirm the fuel flow volume by simply tapping into the pressure test port at the TBI unit.  Hook up a safe fuel hose to a metal canister, and check the actual volume of fuel flow when key ON primes the system or the engine idles.  Pressure alone is not enough, you want volume, too.

Check the base ignition timing with a timing light.  All spark advance is within the ECU;  however, the base spark timing determines the spark advance curve from idle to higher rpm.  Spark could be retarded, which may be due to distributor/rotor misalignment or a MAP sensor/ECU issue.  Verify base timing and check the MAP sensor readouts and function.  Check your vacuum circuits, the vacuum diagrams (PDFs) can be found at our other forum exchanges.  

Manifold vacuum is very telling.  At an idle, you need at least 15 in/hg vacuum, I would like to see more like 18-20 with normal compression and valve timing.  As speed and throttle opening increase, manifold vacuum drops;  however, the base (idle) manifold vacuum helps indicate whether valve opening events (valve timing) are normal.  Ignition timing also affects manifold vacuum:  Retarded ignition timing will drop manifold vacuum and lower performance. 

You may also see a jumpy needle on the vacuum gauge at idle if there is camshaft lobe wear or a compression loss from either valve leaks (the most common cause of a jumpy needle) or a compression issue.  I would check compression or, preferably, run a cylinder leak down test to make sure there is adequate cylinder (valve and ring) seal.  Use the Search box and see "leak down" or "leakdown" coverage here at the forums.

If the valve timing sprockets, chain and tensioner have wear, this could dramatically impact overall engine performance.  Late valve timing caused by timing chain stretch or a defective tensioner will usually create poor performance at lower speeds and have less impact on performance at high rpm.  Regardless, valve timing and valve lift are critical. 

A timing chain installed a tooth off will also drop performance dramatically.  If you suspect the timing chain is off a tooth or there is chain stretch or a tensioner defect, check the valve opening and closing points at different crankshaft positions (following the FSM guidelines).  A worn chain will retard the valve timing and also screw up the distributor/ignition firing point at the rotor.  If you need some tips on how to quickly check valve timing/chain wear, I can assist.

Try these basic tests.  Low manifold vacuum at idle begs a check of the ignition base timing, camshaft/valve timing and lobe wear.  Lobe wear decreases valve lift and causes poor performance.  Everyone wants to "tune out" problems, but if there is something fundamentally wrong with the engine, the EFI and tune-related fixes will not solve the problem.

Start here and let us know what you find...

Moses

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Thanks for the response, so I have set the base timing by getting to TDC and dropping the distributor in as it rotates to the 6 o'clock location. I have also checked timing with a light. Cannot find any spec for it though. Idle timing sits around 10-12 degrees BTDC. which I feel is reasonable. When throttle input is applied the timing does advance. 

I will check fuel volume and manifold vacuum when I get a chance. Though another symptom popped up yesterday as I was looking at it. When I started it it ran fine for a few seconds then sputtered and choked out with smoke coming out of the intake hat. Would not restart right away. I connected the modis and found that the coolant temp was reading 39 degrees. Which it must have been closer to the low 100's. So I did the wiggle test of the CTS. The input would vary a lot. I was able to get it to read properly for a moment and it ran fine when started. Idled for a while and choked out again CTS reading low again. Obviously the ecu is dumping fuel in because it thinks it needs it for cold situations. I have ordered a CTS should be here Tuesday. 

I have a new air filter and have verified that the flexible pipe from air filter housing to t-body is clear. 

I verified the vacuum hoses when I replaced the cylinder head. I'm assuming also the valves aren't leaking since I had new valves installed and head checked by a reliable machine shop. I printed out the vacuum diagram and had it on hand when putting the intake and throttle body back on. Vacuum reading is right around 15 at idle.. The needle vibrates but doesn't sway

As I have replaced the timing set I don't think it's out but I wasn't very confident in my marks. How would I go about checking the valve timing without tear down of the cover?

One last thing I saw yesterday with the scanner. The fuel trims seem to be rich even when the CTS is reading. Scale of 0-256 where 128 is stochiometric reading 30s to 50s which is rich. So I am wondering if my O2 is going out as well. 

Haven't gotten to compression and leak down yet. Though I had done them previously and the only cylinder I had a problem with was the one with the crack in the head. If I remember correctly the leak down for 1-3 was around 15% and compression around 145 with a gauge I believe reads low. Cyl 4 had 85% leak down I think and 70 psi compression. I have not confirmed compression or leak down yet. I'll have to get the tools to do it. 

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wondernad...I like your methodical thoroughness!  Yes, the CTS definitely needs replacement.  As for valves seating, that also takes pushrod length into consideration.  You can have a great cylinder head rebuild and resurfacing that raises the pushrod height and holds valves slightly off their seats.  The vacuum gauge will usually show this as a jumpy, fluctuating needle. 

A cylinder leakdown test would show valve leakage much better than a compression test.  Here at the forums, we have discussed pushrod lengths.  Here's one good example that also applies to your 2.5L four.  There are several other references to "pushrod" or "pushrods" in other exchanges:

Yes, your O2 sensor needs testing at least.  If you do replace it, use the correct NTK or OEM Mopar sensor.  Generic brand sensors are unreliable.

Your earlier leakdown readings were good.  However, I would be concerned about pushrod length after a valve job if the head was resurfaced.  Valve clearances could be well in order, but I would verify to rule this out.  Valve clearance does change as the engine warms, clearance getting even closer and apt to unseat the valves if the pushrods are too long.

If you need to invest in a gauge, opt for the leak down tester.  OTC makes an inexpensive (comparatively) gauge available from Summit Racing and elsewhere.  It's more than adequate for your needs.  There may be similar gauge options at Amazon, read reviews.

Moses

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wondernad...Regarding your initial comment about the tach and diagnostic scanner rpm being far off, there were different speedometers for the 1987-90 YJ Wranglers based on number of cylinders and model year. 

If your tach does not seem accurate, get a reading from an engine analyzer scope or diagnostic tool that picks up rpm from an induction clamp on the #1 spark plug wire.  An industrial infrared rpm tester with pickup tape on the crankshaft pulley is the most accurate.  (This is true engine rpm.)  Compare real rpm with your tach. 

The tach should be unique to a 2.5L TBI model.  If your tach is off, here are the factory part numbers and vehicle application breakouts for the 1987-90 YJ Wrangler models.  A common mistake is the swap of a recycling yard tach from a YJ with a different number of cylinders.  If your tach is accurate, the issue was the diagnostic scanner, set for the wrong number of cylinders or programmed with the wrong software:  

TACHOMETER
56001380  4 Cyl. Engine, 1987
56002914  4 Cyl. Engine, 1988-90
56001565  6 Cyl. Engine, 1987
56002915  6 Cyl. Engine, 1988-90 

Moses

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So I was able to install a new CTS and oxygen sensor. Took a drive while watching scan data. Data still showing it as running rich. Made a small pitstop to ship a package and when I tried to restart it wouldn't until I held the accelerator to the floor. I also experienced a single backfire on slight decel. I borrowed a compression gauge but couldn't get a leak down tester yet. Compression was 140, 148, 149 & 150. Checked fuel flow while cranking engine, fuel came out of supply in steady stream seamed like it was good flow. Had it stalling at the end of my test drive but i believe it is fuel level. Gauge is inop so I fill by mileage. I think one of my kids may have reset my trip meter.

     Still thinking about either a bad CPS or ECU due to rpm reading. With the scan tool at 50 mph in 4th gear it was reading around 1500 tach was around 2300. In my experience it feels like the tach is more accurate. I have another ECU somewhere. Just have to find it to see if anything changes.

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wondernad...You asked early on where the ECU gets the rpm signal:  the CPS.  This signal is very important.  The ICM functions from ECU output and relies on the accuracy and function of the CPS and ECU.

There are other sensors to consider.  Have you tested the MAP, MAT and other values?  I recently addressed the MAT (IAT), which others have found troublesome:

https://forums.4wdmechanix.com/topic/1169-jeep-xj-cherokee-25l-tbi-cps-modifications/?tab=comments#comment-8193

Check out the above exchange...There may be a clue here.

Moses

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