Renix Snap-on erratic readings

[knever3]

I am 13 years into figuring out why my idle is erratic.  I just changed the idle air control again with the '91+ style for the BBK throttle body.  I pulled the IAC and cleaned and measured the resistance.  Idle was low at 550rpm so I adjusted the throttle plate screw a touch and it stayed at 750rpm.  I was watching the rpm on the MT-2500 and it went nuts, rpm and theottle position was all over the place, all at idle in park.  What in the world!  I wiggled the diagnostic connector and the wiring for the sensors and it didn't change.

 

 

 

[hgeranium]

I’m not sure what you’ve tried or tested other than what you’ve explained in this post, so I’ll go into detail just in case there’s something you haven’t checked. Apologies if this entire thing is redundant for you.
 

I see you have an 87 in your signature, I’m assuming other than the throttlebody everything else is still renix. 

 

Have you been having issues since you installed your new throttlebody or even before? I know that the OEM TBs are non-adjustable so I don’t know if that would necessarily cause issues working in conjunction with an OEM throttle position sensor and IACV if it were to be adjusted, especially on an aftermarket unit. 
 

Do you still run the same throttle position sensor as stock? If so it’s very important that you set it to the correct position. If you’re unable to get the correct voltage then it will need replacing.
 

Other than that I would check common vacuum leak points or even a smoke test if you have a machine by chance. The biggest ones are definitely the map sensor to the throttlebody, EGR (vac lines and EGR gasket itself), and a bad manifold gasket. Other than that, it should just be the fuel pressure regulator vacuum and the PCV vacuum. it would be good to check them all anyways. I know I had a minor vacuum leak when chasing down my weird idle at the throttlebody gasket as well
 

Speaking of your MAP sensor, have you tested that? Correct vacuum is very important for its function but it could also be faulty on top of that.

 

does it drive fine or will the bad idle go away? Is it consistent or intermittent? My erratic idle was usually only in park, and shifting out of it fixed it (if you have an auto ofc). 
 

This is most likely not part of the issue, but does your fuel pressure look OK? It should be about 31 at idle and 38 to 39 without vacuum to the fuel pressure regulator. My truck was running super weird when my fuel pump was in the process of going out so I thought I’d mention it.

 

Also, most importantly. Have you checked all of your grounds? Cruiser has a great writeup on testing and upgrading all your grounds. It’s especially problematic at the C101 connector and at the harness parallel to the valve cover. It certainly wouldn’t hurt to check and upgrade the crappy fuel pump ground.

I’m very well-versed with your issue, mine seems to be in remission so far. If I could be of any more help or if you need pics let me know.

 

[hgeranium]

Also, have you happened to fix the issue with crankcase oil getting pulled into the air box through the CCV vent hose? I know that caused quite a lot of issues by ruining my throttle body gasket and fouling up my IACV over time. 

[knever3]

I really enjoy all the effort you put into your post, sorry I didn't provide the details.

 

'87 Comanche that was a manual 2wd originally.

 

I swapped in an automatic and TCM and ECM from a '90 as well as the engine bay harness eliminating the C101 connector.  I have a '94 block (bored 0.60 over and KB forged pistons with stock 4.0 rods making it a 4.7L stroker) a '96 head, a '99 intake and an HO BBK throttle body, '95 fuel rail and fuel pressure regulator.  I can't remember my running fuel pressure at the moment.  I have 21lb/hr fuel injectors if I remember correctly.  EGR deleted.  This is of course running the '90 Renix engine control.  

 

I have run a smoke test recently, the throttle body bearings have a slight leak, but not severe.  The crazy part is the readings with the MT-2500.  At idle all of the readings are all over the place, I can't begin to troubleshoot if that is the case.  Strange thing is, it seemed to be idling just fine, but the RPM, throttle position, manifold vacuum etc. is all over the place!!  This morning I remote started it and heard it surge up and down the whole time it was running, mind you it's -11°F this morning.  I would like to see what the IAC motor is doing one of these days, but if the scanner is not giving me the correct information then all is lost.

 

On a separate matter, my alternator doesn't seem to charge at idle.  I had recently replaced my battery because of hard starting, it had a bad cell and was not keeping a charge.  I noticed that off idle my headlights would dim, then get brighter when I just moved off idle.  My idle was low, at 550 rpm, so that might be normal for an alternator to no provide a charge.  All I know if if I remote start it and run it for the desired time it's just draining the battery for the 20 minutes, and for my 10 minute drive it has to hurry up and charge it back.  I don't have a long drive to work.  I adjusted my throttle plate screw to get up to the required 750 RPM, but that should be the IAC motor's job.  The throttle plate screw should be just a stop when the butterfly is closed against the body.  

 

Oy

[Ωhm]

NickInTime has a short utube on the MT2500 for RENIX. Mentions Keep-Alive Battery and something about bootlegged cartridges. Other than that, I'd look at power and grounds for MT2500. Could be crazy serial data output from the ECU if MT2500 proves good.

 

https://www.youtube.com/watch?v=qPV5pEGUHWw

[knever3]

Funny you mention that, the 9v keep alive battery was bad when I pulled it out of storage, after replacing it with a fresh battery the readings were erratic.  Hmm....  

 

I am wondering if I am picking the correct options.  I have an '87 which used to be a manual with no A/C, but now the ECU is from a '90.  I wonder if the setup I just picked is wrong.

 

My truck runs exactly like that, maybe just a stroker/renix thing.  I may be asking too much for a steady idle.

[knever3]

I think my hunch was correct on having the wrong information entered into the scanner.  JTMW6416HT173200 is my VIN, but entering it in the decoder doesn't do anything.  If T is my 10th digit on the scanner it says my truck is a 1996.  I selected 1987 and 1990 since I don't have the VIN for the 1990 XJ I grabbed the Auto computer and TCM from.  Anyway, it works again.  I must be missing a digit before the J, but from my picture I can't seem to find one there.  I did some researching and it seems like I am missing the "1" first digit which didn't show up on my picture I took somehow.

 

I had Autozone use some cheap meter to see if my alternator was charging, but I don't trust it.  If I turn my headlights and fog lights on my battery voltage goes down to 13.2v.  I think the alternator is not putting out enough amperage to keep the voltage up at low idle.  I don't really feel like taking it out to have it bench tested, but I don't have any way to load test it other than measuring voltage while some accessories are on.

[Ωhm]

54 minutes ago, knever3 said:

Anyway, it works again

 

[hgeranium]

6 hours ago, knever3 said:

I had Autozone use some cheap meter to see if my alternator was charging, but I don't trust it.  If I turn my headlights and fog lights on my battery voltage goes down to 13.2v.  I think the alternator is not putting out enough amperage to keep the voltage up at low idle.  I don't really feel like taking it out to have it bench tested, but I don't have any way to load test it other than measuring voltage while some accessories are on.

I've had horrible luck with the parts store reman alternators. I'm on my third one, luckily it has a lifetime warranty, but the quality and lifespan is such a mixed bag.

 

Next time it goes out I'm planning on getting a brand new unit from RockAuto. That seems to be the only way to assume you're not getting a beat up and used alternator with a fresh coat of paint. I've already been screwed over and stranded thrice because of an alternator, so I'd recommend just picking one up as collateral. Rockauto has a brand new 100 amp ACDelco unit for $80, no core charge.

[knever3]

My current alternator (See what I did there...) is a completely new Bosch 100A alternator.  I think I am going to add a volt meter and hall effect ammeter to my truck, they are pretty cheap.  I'll just put the sensor on my alternator charge wire and can see what float voltage is, charging voltage and current at any given scenario.  The Autozone static test that gave me a "ok" print out was a joke, especially after the idle was at 1100rpm's at the time.

 

Something like this:

 

https://www.amazon.com/bayite-Waterproof-Display-Voltmeter-Transformer/dp/B0BFJ5NV5L/ref=sr_1_2_sspa?dib=eyJ2IjoiMSJ9.aJvoGqVcTh33MrpnnbjyqAVDWW0thAKt7BT1eFhIKylxLS7bd_ITvqkz9bv_9hIoJ6OyYj568hxDaaA0PZJRICjtvA9WtB-hX1MSYi_ivKUTUQFRwQTw67dkjNpBaO0raO6jJoLnU0qzPBNZnpIIM9H1xlQ_-jXYmtFLb26Pa419rHjVQqKw7Kayk4oAaUQT7pEuZjJmjW77Q8QPqmDm3Y8AgzpwXOT1csQ2ZYEUXpQ.xmgRCQSZ3Nbtp0CkEU4uGFIlLiQYkfHNo1R-D3In9Go&dib_tag=se&keywords=hall+effect+current+sensor&qid=1770061054&sr=8-2-spons&sp_csd=d2lkZ2V0TmFtZT1zcF9hdGY&psc=1

 

 

 

 

[hgeranium]

Any possibility that you have parasitic draw coming from somewhere? Sound system maybe?

[watchamakalit]

don't trust the stock dash gauges to show accurate readings. 

[derf]

32 minutes ago, watchamakalit said:

don't trust the stock dash gauges to show accurate readings. 

That's still true to this day.  My 2021 Wrangler has both a speedometer dial and a numerical readout.  My dash will show 70 on the numerical display but the speedometer dial will be 68-69 or so.  And they're being run by the same sensor data.