Intermittent performance problem

Lately I've noticed that my 89 renix has developed a problem that mostly appears when the engine gets up to operating temp.Originally I thought I had gotten some bad gas so I dumped stp and some octane booster in the tank but the problem is still there.I did a fuel pressure test today and at idle the gauge reads 29psi then it goes up to 38psi with the line disconnected to the fuel pressure regulator.Seems a few psi low compared to spec.The fuel pump is new so I dont know why it would read a little low on fuel pressure....it's only -1 psi compared to spec,right?

**The problem shows up as a miss right off idle (but never completely stalls)then clears up around 1500rpm and seems like it's happening after driving for a half hour or so when it gets up to 210-220 * Kind of like it's miss firing or a bad plug but why doesn't it do it all the time?**

Tested the tps also and it's good(4.88v input and .847v output which is 17%)no dead spot.
Took of dist cap and it's fine.

Checked for vac leaks and everything is fine there....egr valve is disconnected so thats not the problem either.

Here is a list of things that have been replaced with new parts in the last 2500 miles:
new engine,fuel pump and filter,plugs,injectors,knock sensor,manifold air temp sensor,cts,o2 sensor,cps,ignition switch, and exhaust manifold(not cracked) so I think these things can be ruled out because all this was done during the motor change and ran fine untill lately.

Could this be an ignition problem that only crops up when the coil gets hot or something?How do I tell if I have a misfire?

The dist ,cam sensor,coil,map sensor,iac,and icm are the parts that didn't get replaced during the rebuild because I ran out of funds.....

Help!!!
Thanks

Re: intermitten performance problem

have u checked vacuum?
did u replace vacuum lines when u replaced engine.

Re: intermitten performance problem

Yea all the connections are tight and vacuum is steady on the gauge at about 18.The blimp behind the bumper is new and all the connections are solid

Re: intermitten performance problem

Thanks for the quick reply...

Re: intermitten performance problem

how about the temp sensor on the back of the block?

Re: intermitten performance problem

On the renix that one is just for the temp gauge.

Re: intermitten performance problem

ok I know one hooks to the puter.

Re: intermitten performance problem

The one on the lower side of the block,below the intake is were the ecm(puter' lol)get's info from.

It's new.....

Re: intermitten performance problem

But I'm probably going to validate it tommorrow with the dvm.

Re: intermitten performance problem

Drove it last night to go get some heet gas drier and for the 10 minute drive to the store it ran perfect.Went inside for 5 min. and started it up and put it into 1rst,let out the clutch and it started to stumble.Drove to the first stop light and when it turned green let out the clutch and NO STUMBLE?It doesn't backfire at all and never completely stalls.

Like I said it happens right off idle and up to 1500-1800rpm then clears up?
Anyone want to take a shot in the dark here??

Re: intermitten performance problem

You might try replacing your rotor with a heavy duty NAPA brand or similar. I had replaced an entire ign.set on a stick vehicle once(an '81 BMW 320i) and it would run fine as yours 'til it warmed.It would run a little rough between 2nd and 3rd while trying to maintain a constant speed. It either bogged in 3rd or stuttered and stammered in 2nd! Later that week I was returning to my home in TX coming out of Louisiana and going along at hwy speed the engine just died!
I was on the outskirts of a very small town at least 150 miles from my home and car wouldn't restart! Long story short, the rotor popped between the tab that contacts the cap tabs and the spring metal that touches the center distro-cap carbon pin! A Hwy resident gave me a ride to town Napa store and lucky for me they had 1 rotor for my vehicle. The quality of the Napa rotor was far better than "brand A-Zone"! That also cured the low speed problem between 2nd and 3rd! Mileage increased as did performance.
Forgive my beemer exploits, it was my back-up because my '89 Cherokee was out of commission temporarily!!!
Hope my suggestion helps!:woohoo:

Also I've read that you should use an analog (non- Digital volt meter) Multimeter to get a proper reading on the TPS. You may have one on the verge of failure!

Re: intermitten performance problem

You might try replacing your rotor with a heavy duty NAPA brand or similar. I had replaced an entire ign.set on a stick vehicle once(an '81 BMW 320i) and it would run fine as yours 'til it warmed.It would run a little rough between 2nd and 3rd while trying to maintain a constant speed. It either bogged in 3rd or stuttered and stammered in 2nd! Later that week I was returning to my home in TX coming out of Louisiana and going along at hwy speed the engine just died!
I was on the outskirts of a very small town at least 150 miles from my home and car wouldn't restart! Long story short, the rotor popped between the tab that contacts the cap tabs and the spring metal that touches the center distro-cap carbon pin! A Hwy resident gave me a ride to town Napa store and lucky for me they had 1 rotor for my vehicle. The quality of the Napa rotor was far better than "brand A-Zone"! That also cured the low speed problem between 2nd and 3rd! Mileage increased as did performance.
Forgive my beemer exploits, it was my back-up because my '89 Cherokee was out of commission temporarily!!!
Hope my suggestion helps!:woohoo:

tps tests out fine.....I think my cheapo plug wires are misfiring.