Cyriac

Hello everyone,

Big frustrating problem on my 89 4.0 5-speed.

I have a recurring very high RPM issue that I'm positive is caused by a sensor or a least is electronic in origin...

Sometimes it drives fine.

Sometimes and on longer drives it will stall when going into neutral and won't turn on again. If I manage to start it before coming to a stop by starting it in gear, rpms will rush to 2000+ slowly rising even more.

If I shut it off then, either it well restart exactly the same way (very high idle) or no start at all. Even after cooling a little off...

Yesterday I had to wheel in fields but the issue hadn't resolved so I was crawling up and down hills in 4lo 1st gear... Then after around 15mins or so, without doing anything and at a standstill, rpms just went back to normal while running... Just like that! And the rest of the day was fine. Never raised by itself and when stalling it would start fine, normal idle.
But today, I stalled when parking in front of the post office and when I started again, out of nowhere it did its high idle routine and shot straight up to 2500...
I didn't have a choice and had to go to work like that... When I arrived 15mins later it was at 3000rpm idle... We'll see tonight in 8 hours...

Info on this 4.0:
I have completely cleaned and replaced all TB sensors and IAC and adjusted the TPS with a multi meter and IAC pintle properly. So what could electronically be giving a bad signal???
Can't be a vacuum leak because otherwise it would occur all the time. Perhaps I need to inspect everything closer, never really had the opportunity to...

Could this be linked to engine temperature? It's never overheating and the heater works and oil pressure is good.

Any ideas? Could it be one of the sensors I haven't replaced yet? Coolant temperature? Knock? Crankshaft position??

Thanks in advance for your input!

cruiser54

tPS is probably going bad.

But, before you go replacing parts, go to www.cruiser54.com and do Tips 1,3,4, and 5.

Cyriac

Thanks Crusier, I had hoped you'd chime in! :-)

Well the TPS itself is new, I fully restored the TB and all related sensors... Maybe the connector is bad? I adjusted it following your procedure... Would the TPS alone be accountable for all my issues?

BTW I don't know if you remember but I'm the guy who tried to solve the bad ground issues a few years ago and you commented something like you had never seen such dedication in fixing that problem! ;-)

I finally got the gauges working by wiggling the fuse... The female receiver inside the fuse block is half broken...

Anyway, you don't believe my associated no start issue could be a CPS? I cleaned it with brake cleaner when I replaced the clutch slave cylinder...

cruiser54

I remember you!!

What did you do with the IAC pintle?

And yes, this definitely points to TPS/IAC issues.

Cyriac

Oh OK, I'll look into it then. For the pintle I adjusted by pulling the sleeve back and unscrewing the tip. Made sure not to force the IAC with the mounting screws too. I think... Idle does fluctuate between 1100 and 400 rpm on different days. I don't know what it depends on... It's all very random...

On a side note, do you think the early HOs are better? 92-96?

LiamLikeNeeson

Omg I wish I had an 89 4.0 5 speed!! I have an 89 4.0 auto 4 speed but damn that's awesome, regardless of any issues it has is still love one

cruiser54

The IAC is probably bad since they are very sensitive to what you did to it....

Please clarify your question about the HOs.

Cyriac

But to be sure, when you get a new IAC you need to adjust the end to fit snuggly, right? I understand that it shouldn't be forced in at all but we agree that the pintle should be adjusted?


Well I don't really want to start a huge renix vs HO debate but it seems to me that Renix 4.0s have very archaic electronics and a bunch of sensors not seen in later engines. I suppose HOs come with their own problems but I just imagine less could go wrong?

Basically are HOs better engineered than Renix 4.0s? I suppose bad wiring and poor resistance through the ages cause most of the problems on older 4.0s?

cruiser54

DO NOT mess with the IAC pintle. The system moves it in and out according to inputs.

Renix is not archaic. works fine if a few issues are addressed. HO is not without issues.

The HO's wiring system is no better in alot of ways and they're getting old also. A 90 REnix versus a 91 HO as far as wire age?

Have you visited my website?

Turbo X_J

Manual idle adjustment went the way of the carburetor.

Cyriac

Oh so when I get a new IAC I just mount it in straight out of the box?

So you're saying that none today are without flaws... I guess that is natural for 20+ year old vehicles!

I have checked out your site as well as your tips, I'll get around to it! :-)

cruiser54

Mount the IAC right out of the box.

Get going on the tips. 1,3,4, and 5 first.

Cyriac

Understood.

I just had the problem, on a cold start, so I decided to tinker with the connectors, I unplugged the TPS and the rpms went back to normal... Plugged it back in and unplugged the IAC just temporarily to be sure rpms wouldn't rise uncontrollably.

I hope I'll get to the bottom of this. Can't check right now but when I do I'll post the results here.

cruiser54

Get the tips done.

Michael Luce

Isnt the TPS on the 89s adjustable?