Erratic high idle help, please!

[ActuallyaMazda]

Hi everyone! I’ve been attempting to hunt down a high idle issue for a week now and I thought I’d post my progress here for some help if possible!
I drive a ‘94 Mazda B4000 4x4 with a 4.0 OHV and an A4LD transmission and the previous owner messed with a bunch of stuff that I’ve been trying to correct over the past year since I bought it.
A week ago today, I cleaned my throttle body with a throttle body safe cleaner we had at the shop that I work at and the issue started when I cranked the vehicle to go home.
Upon startup, the engine started revving like it was a runaway diesel. It held at about 3000rpm while I had it in park and then when I put it in reverse to back out of the shop it dropped down to about 900 where it usually is in reverse. Drive gears is the same way in that it operates seemingly correct under load and at red lights. Even when it is up to temperature, if it is in park or neutral, it idles anywhere from 2000 - 3000rpm.
I have so far replaced the throttle body with a low mileage one from the junkyard, Throttle position sensor, IAC valve, and PCM relay in the last few days after hunting for external vacuum leaks and not finding any using both smoke and starting fluid. I replaced the FPR a couple months ago and thought that it could potentially be an issue with the diaphragm since there are intermittent backfiring issues on warm startup. Vacuum line to Transmission vacuum regulator is all good to the regulator as well. I also replaced two ground bolts that were corroded at G200 and G103.
It sometimes will idle down when cold after shooting to 3k and act like it’s going to run ok and then it shoots back up to at least 2k.
Anyone have any ideas on other things to test/replace? The thing sat out in the woods for god knows how long before I got it and if it needs to be replaced I have no problem doing so.

 

[franklin2]

Have you ever messed with the throttle stop screw? If it's totally warmed up and idling, what happens when you unplug the idle air valve connector?

 

[ActuallyaMazda]

The new throttle body that I installed has not had the screw adjusted and if I pull the connector at the IAC when warm, it drops the idle, if it’s cold then it stalls the engine completely.

 

[franklin2]

What is your guess at the idle speed when warm and you unplug the connector? It should be very slow, almost but not quite stalling.

 

[ActuallyaMazda]

My best guess is about 700 - 800. I’ll warm it up when I get home, but it’s noticeably slower than what it normally sits at before all this started which is about 1000 - 1100rpm when warm and in park/neutral.

 

[franklin2]

I would adjust it slower. Take it for a drive and make sure it's warmed up fully. I would set the idle with the IAC unplugged closer to 400-500rpm. This will also make sure the TPS sensor voltage is below 1 volt at idle.

 

[ActuallyaMazda]

I will try that when I get home. I should also mention that when turning off and starting back up when warm, it starts really rough and I can hear what seems like a backfire from the air filter for about 30 seconds. I’m not sure if that has any bearing on anything related to the IAC or TPS, but that has been a symptom that I installed new spark plugs to try and help last night and it did it again today. Could adjusting that idle screw help with that as well?

 

[tw205]

All this atarted after the throttle body cleaner? If it's the same as my 95 4.0, the EGR tube enters the plenum immediately after the throttle body. The port for the tube has a rubber grommet that seals the tube to the plenum. That seal may have been sealed before you cleaned the throttle body but the cleaner did its job and cleaned the carbon around the seal and it now leaks. Mine did the same thing you are describing. Erratic idle and all. Did you smoke test it? The tube is covered in a fabric material making the smoke difficult to see. Hope this helps.

 

[ActuallyaMazda]

Any insight definitely helps, thank you! Is that EGR tube coming off the throttle body assembly itself or is it coming off the intake manifold? There is one vacuum port on the throttle body that I see (it’s the same on the old one as well as the newer one) and that is capped off with a vacuum cap, so I at least know it isn’t leaking from there. It’s an off-road rig and the previous owner removed pretty much everything emissions control related except for the PCV valve which I have verified works properly and isn’t the source of any leak. I haven’t properly smoke tested anything because I didn’t have access to the equipment today, but I’ve been checking vacuum lines with starting fluid to see if there is any increase in rpm to no avail.

 

[ActuallyaMazda]

And yes it was fine when I went to work last Saturday and then I cleaned it and it immediately started when I started to go home. And the throttle body that is on there now is a new one that I verified hasn’t been cleaned by all the healthy gunk around the Teflon seal inside that I’m assuming I wiped away when I cleaned the TB.

 

[tw205]

EGR tube enters the intake manifold immediately after the throttle body. It's a steel tube from the exhaust ( exhaust gas recirculation) it enters on the bottom side of the manifold. My rig is a beater and I don't have state inspections so I pulled the tube out and put a rubber expansion bolt in the hole and my trouble stopped. I had done the same thing you did. Cleaned my throttle body with throttle body cleaner. Took me a week to figure it out.

 

[tw205]

It's a steel tube about an inch in diameter. Easy to spot.

 

[ActuallyaMazda]

I’ll look for that, but I’ve never seen that before on my rig even when I did my intake manifold gaskets! I’ll look and see if the previous owner had capped it and I did something to it with the cleaning. I don’t have state inspections either so if that is the case, then I’ll be very happy. The exhaust manifolds look to be stock and I haven’t seen anywhere that tubes could coming off of them except for what looks like a heat shield with a huge hole maybe 2 1/2 inches wide on the passenger side manifold towards the front of the engine. Would that be where it would’ve been if it had it?

 

[tw205]

It will be on the driver side I can send a picture in the morning. I live in eastern time zone so it's a bit late here. If the PO removed all the egr equipment he had to seal the port with something. If yours is like mine.

 

[tw205]

When I look your truck and egr parts on rockauto they look the same as mine or at least very close.