Need Advice-next steps to fix poor performance

Need Advice on next steps to solve poor performance

I have a friend who bought an 88 for $800 with only 140K. It is not perfoming like it should. Here is what I have found

After starting it up, the idle tends to hunt for a while, like the A/C was being turned on and off. So the RPM readings are between 1300 and 900. it settles at about 1000.

When taken on the road, it accelerates OK but lacks the typical power. When climbing a hill, it is really slow in acceleration and doesn’t down shift like I think it should. Manually putting it in 1-2 helps but doesn’t really solve the problem.

He was complaining about the performance soon after he bought it. I suggested he clean the TB thoroughly, which he did. He did not adjust the TPS. He put on a glass pack and a new filter. The vehicle did pass emissions testing.

I tried to adjust the TPS but found that when probing the three wire connector, I could not get a voltage reading for the out put! Got on-line and found the thread saying to probe the TCU connector instead. Found 4.64V on input and was only able to get to 4.15 at maximum adjustment. This did not fix the problem.

My next steps would be to replace the AIS, TPS and CPS.

Oh, the vacuum reading is about 17.

Any recommendations on what the next steps should be?

It sounds like a vac leak with the hunting, check the egr, they can be a real PIA sometimes. Did he do the complete 'new to me' maintenance, plugs, cap, rotor, wires, fluids, dump a couple of bottles of Chevron Techron in two tanks ??
It is also possible it could be the cat partially blocked with loose stuff that moves around in there, drifts to the back when heading up hill. Will it manually drop a gear ?? Will it kick into passing gear ? does it have one of those comfort/performance switches ?? How long the O2 sensor, original maybe ?? Might be time for that too...

With vac at 17, I don't think there is a leak.

he did cap, rotor, plugs, wires. He has not run a tank yet - to 'new"

Since it passed emissions, I assume the cat and o2 are good. I own a 90 and am very familiar with the systm. The trims only have a 25% range compared to 35% for 91 and newer. So the computer apears to be controling the a/f OK.

I am not sure about downshifts. Upshifts are ok. What is your thinking on downshifts? Mine is that the TPS is in questions since the 83% voltage delta can't bee achieved. I am not convinced that the AIS is any good either.

This may be a dumb question, but does he have larger than stock tires on it? If so, have the gears been changed?

No question is dumb! It is totally stock. I am wondering if the TPS cannot be adjusted to within the 83% spec, is it bad?

BTW: the Transmission Cable was out of adjustment. It helped in downshifting but not the root problem.

I had my tps on my 90 renix set to 83% exactly. I spent countless hours trying to fix the hunting idle issue and finally on a whim set my tps output to 4.2 volts ( per an AW4 manual) . Combined with my 4.64 V in thaty puts my tps at 10 %+/- . BAM the xj is jack the bear . I have read here about people with the renix tps taking it to a dealer and them stating that the 83 percent deal is ball parkish at best . This is what I think . In an ideal world the tps input would be 5 volts exactly . so at 4.2 V its at about 84% . is this where the 83% comes from ? Im just guessing but the 4.2 volt ouput improved my issues no matter where they were in the power range . This is all just my .02 of course and all applicable warnings apply......

ARP,
Thanks for the input. My mechanic suggested less than 12% difference which syncs with your comments. I'll give it a try.

New TPS from Autozone ($65) fixed the high idle and performance. The idels still hunts within 500 RPM. The AIS is suspect.

You could always try removing the air intake and the air stepper motor .( un plug it after a shutdown ) and find a suitable airtite substance to cover the stepper motor hole left on the top of the throttle body( a buddies thumb works best ) start engine then listen to see if it still hunts. At this point you can bench set the base idle with the " do not mess with screw " although I think that just masks the truer problems. jumping of 500 rpm flutter is a lot in my opinion , that said though,it should be easier to trace then a say +\- 100 rpm flutter. Although the tps didnt solve your hunt issue, your further ahead in the performace end of things... Ive learned with my rig to just " get use to" some things " after all its 14 years old .I was driving myself crazy always listening for weird sounds etc . Now I actually turn my stereo up again shot in the dark unplug your injecters one at a time to see if they are are contributing a full 1/6th each at idle. Have you regrounded every gorund you can find? Unfotunatley the list may be endless...

This is an automatic correct? When was the transmission fluid last changed?

Copperhead

ARP thanks for the info. May try that out. I suspect the AIS because everything was really "gunked" up before cleaning. I am not convinced that the AIS is working properly after fighting the carbon build up and being 14 years old.
Before changing the TPS, I adjusted the transmission cable, which did have a positive result, although small. After the TPS replacement, I was not completely happy with the shifting, but it was within the tolerance zone of acceptablity for me.

Copperhead. I know the fluid has not been changed recently. Given that only 4 of 16 qts gets changed when you do it. I am not convinced that changing the 4 qts would be significant. Doing a complete flush may be and should probably be done.

Disco the battery and remove the IAS and clean it good with carb cleaner. Also hit the seat inside the TB where you pulled the IAS. The next thing I would do is to run a can of BG44K fuel system cleaner. Don't use the cheap crap, it doesn't work.

While your 17 inches of vacuum sounds fine, double check for leaks and make sure the MAP is connected correctly. I am starting to see a lot of leaky brake boosters. Pull the hose to the booster and plug it and see if the idle still hunts.

Surprisingly a bad temp sensor or O2 sensor can cause the hunting as well. After the motor has run a while and it thinks it should be hot enough, the computer switches from a preprogrammed mode to a closed loop mode in which it uses the sensors to try and adjust everything. When it goes into closed loop mode, if a sensor is way out of line, it will switch back to preprogrammed mode for a while and then try again.

Great info old_man! Will check out vac and AIS cleaning that was done. I believe the O2 is ok as it did pass Emission Testing, which is fairly stringent. If I have to I will test it also.

Which temp sensor are you referring to, water or air or both?

Any ideas on what impacts shift points for the trans? I did adjust the cable and made sure the connection to the TPS was good.

I know some of this has been posted but being an owner of a 88 XJ I do have some insight to replacing the stuff time has comsumed.

Like any used vehicle without maintenance records I would do all the 30,000 mile and 50,000 mile maintenance so you know it is zero timed this week.

The 4.0 engine is known for the cranshaft damper/haromonic balancer going bad. Mine went bad and began to walk forward. When I had it replaced I noticed more acceleration. I recommend you change the fan belt. While you have the belt off check each pulley to see if it turn easy. Change out the component that turns hard. Also change out the idle pulley by the AC compressor, they run $10-15 and is cheap insurance. You may also want to install a new fan clutch for the radiator, they degrade after 5 years and it prob theo stock one so it over 16 years old. Figure $30 for the belt, $30-$60 for the fan clutch. Do a search lots of posts in this subject.

I hope it came with the owner's manual. I would do a tune up - cap, rotor, wires, and spark plugs. Buy these at the dealer, figure $60-$70 for those items.

Replace the Crankcase ventilation system. It the large and small hose assemblies connected to the valve cover. These are 50K mile items. figure on $100 for the small tube and the two halves of the large tube. This will help any vacuum leak you have. These you buy at the dealer also.

If the vehicle has the "part time" transfercase then pick up the front axle shift motor harness. Every time you change the oil filter you drip oil on the harness, after a while it no longer seals on the shift motor. It will not shift in to 4x4. If you have a 2 wheel drive then disregard. This is a $10 dealer part.

Clean the throttle body - Even if you did this I doubt the throttle body was removed, cleaned and installed with a new gasket. I took my XJ to the dealer for a throttle body cleaning. I stood there and watched the guy do it. He showed me to keep spraying cleaner through the hole next to the large bore until no more brown comes out. The square opening is the Idle air controller passage. After he had it reassembled he hooked up the test set and checked over the entire engine, adjusted my TPS into range and adjusted my idle speed. I think that was the best 1.0 hours of dealer labor I had ever paid for.

Exhaust system - you have good vacuum so doubt the cat coverter is bad but it may be close to the end of it life. Is the cat as long as the muffler or is it shorter? If it the length of the muffler then it the factory cat, keep that in mind for the future. I had my cat changed at 125K miles and what a difference it made, like taking a cork out of the tail pipe.

Once you change the fan belt, have the throttle body professionally cleaned and do a tune up see how things go. There are several sensors which control how the engine runs, they are:
* Throttle position sensor
* Manifold Abs pressure (MAP) sensor
* Inlet air temp sensor - in the intake
* Engine knock sensor - on the side of the block, just above the oil pan

I'd recommend replacing the sensors in the order I listed. The XJ will run just a little better with every new sensor. Your knock sensor could be messing up and retarding your timing, that why you have no power up hill. Don't forget a new air filter. If you have oil on the air filter then changing those valve cover tubes should fix it.

Sorry meant to say flush. My wife and I inherited a vehicle with an auto tranny, (we never owned one before). Had similar problems with hunting, and a flush made a major difference. Didn't figure it was the complete source for the problem, but figured it was one more thing to check.

Copperhead

sjx40250 said:

Copperhead. I know the fluid has not been changed recently. Given that only 4 of 16 qts gets changed when you do it. I am not convinced that changing the 4 qts would be significant. Doing a complete flush may be and should probably be done.

Click to expand...

Martin and Copperhead,

thanks for the additional info.

Having owned a 90 for 14 years, I am somewhat familiar with a lot of the issues. The benefit is that I addressed each one as it came along and have not experienced some of the issues this rig has.

This rig was bought by a friend of mine who had no prior experience. I would say that we have checked 80-85% of the items Martin listed and are going through the rest of the items. I am very appreciative of your taking the time to list and explain. I learn a great deal from the contibutors to this forum. Thanks.

i just responded to a post by "fletch" you may want to give it a try. 87-89's had problems with ignition crossfire, it can hurt to check it out.

Was having trouble, getting a constant reading on my TPS (tranny). On a whim I tried testing the voltages on both side of the connector. No apparent corrosion, but I´d guess a little film of oil or something, cleaned the connector and things settled in pretty nice. I´m now getting the same voltages on both sides of the connector.
Also found 0.8 standing voltage in the ground circuit, some of the resitance seems to be in the TCU, I tested two. Checked the wiring/connectors, pretty close.
Finally found a resistance in the TCU ground circuit, the grounds at the dipstick holder, the one for the ECU was soldered to the ring connector, the one from the TCU was just crimped and green. Replaced the ring connector (soldered) and ran an extra TCU ground to the firewall from the dipstick holder. Helped some.
Have some vacume issues to clean up, intake and such.
Afterwords, I´m gonna try setting the output value of the TPS at the 4.2 mark (like suggested), idea crossed my mind before. My imput voltage to the TPS was 4.62 from the TCU (checked all wiring and connectors/no resisitance), Guess the slightly low voltage, from the TCU (is the nature of the beast), could this be a contributing factor?
Shifts are pretty good now, I can´t seem to leave well enough alone.