Have an 88 NA. Didn't start doing this until recently.

On cold start, the AWS does what it's supposed to and holds the rpms at 3k for a bit. When it comes down from 3k though, instead of holding idle like it should, rpms just keep dropping and the car dies.

I have to keep giving it a bit of throttle to keep it alive for about 2 minutes. After about 2 minutes, it holds idle on its own.

I was thinking it may be the TPS? Any suggestions would be appreciated here.

 

Is the sensor on the back of the water pump housing plugged in? It looks like an injector connector, sort of below the alternator. It can do what you're describing.

Otherwise checking the TPS is a good idea. The thermowax could cause this issue too, but usually it would fail in the opposite way and idle will never come down.

 

That little guy?

 

That's the one.

It's plugged in in the picture, but if unplugged or if the pins are dirty the ECU can't read the coolant temp. When this happens the ECU assumes the car is warmed up and won't enrich the mixture like it normally should for a cold start.

 

I'll try unplugging it and cleaning the pins. There's 3 coolant temp sensors I'm aware of. The one mentioned above, the one on the bottom of the radiator, and the one below the oil filter. Do all 3 of these affect cold start?

 

The one on the bottom of the radiator is a temp switch, it's normally closed and then opens at 70 something degrees water temp (I think). It determines whether the AWS does the irritating 3000rpm startup, and also has some sort of function with regards to the air control valve but I don't really recall what exactly. In my experience the ECU doesn't really care if it's not plugged in, the 3000rpm start up is to light off the cats and isn't necessary to idle the car.

The one below the oil filter is the sender for the coolant gauge. It has no actual engine control functions.

 

Gotcha!

I took the connector on the back of the water pump off and gave it a good cleaning. Still has the cold start issue. I read somewhere that a bad or sticking AFM may cause what I'm experiencing. Think that's a likely culprit? Or most likely the TPS?

 

The AFM is unlikely as a culprit, they're fairly reliable.

The TPS is a good thing to check next because they're known to go bad. You can search but basically with the car fully warmed up and key to the ON position, two of the pins (green and black, I think) should read 1V with a multimeter. You can adjust it using the screw on the throttle linkage. Then press the throttle linkage slowly and make sure the reading moves smoothly with no jumping around.

 

Understood. Going to adjust the TPS and see where that gets me. If that doesn't work, I'm at a loss

 

Reopening this thread for a moment because I've made a discovery. I have no idle adjustment screw. After buying the tps tester from Banzai, I went to my car to adjust after having driven it around for about 20 minutes. Plug in the tester, and get my screwdriver handy, and boom. No idle screw. Guess that explains my problem. However, therein lies my question. There are 2 other screws around the thottle and throttle body. What are these for? The first pic is of the non-existent idle screw, but the other two? Also, any pointers as to where I can find another idle screw would be appreciated.

 

The one on top of the throttle body that is designed for a flat head screwdriver is the idle screw. It lets you set the idle where you want it by using a little bypass passage.

The one on the linkage with the spring is your throttle stop. Don't adjust that, it isn't designed to be used to set idle. Closing that down will prop the throttle blades open at idle, which is not the correct way to do this. It also changes the TPS reading since it changes the resting position of the throttle blades.

The missing one is the TPS adjustment as you already know, and that is probably the cause of your idle issues since the car is going to see throttle at times your idling (or vice-versa). You should replace that screw since you definitely need that to idle properly.

EDIT: As for where to find the TPS set screw, you could try finding a used throttle body and taking it from that. Otherwise if you don't care about it looking OEM, most of the threads on this car are common metric threads. I think it's smaller than M6 but it definitely isn't larger, so you could try getting a few machine screws in assorted metric sizes at the hardware store and see which one fits.

 

I ended up adjusting the screw on the linkage with the spring before asking on this thread. Any particular way to get it back to the way it was?

Did some digging and found the adjustment screw and spring from Mazdatrix. $10 for OEM.

 

I think that there may be a factory spec for the throttle stop in the FSM but I'm not 100% sure. All I know is that normally it isn't advised to change the throttle stops. The factory adjustment for the primary throttle (the lower one) is given in the FSM, but I'm not sure if the secondaries (upper ones) are given a spec.

If there is no factory spec given I would take off the plastic intake elbow, hold open the secondary blades (the top ones closest to the passenger side that open in a pair) and look down the bores at the pair farther down. Those are the ones you've adjusted with that screw.

On my car at 0% throttle they are all the way closed so you should be able to adjust the stop until they just touch the bore. The primary (bottom) blade is usually open a tad at 0% throttle because of the thermowax system, and it slowly closes down as the car warms up, so don't try to adjust that one since it's normal.

 

Went ahead and fixed the one I accidentally adjusted. Any idea what the stock thread pitch is for the idle screw?

 

I don't know unfortunately, I've never had cause to remove them myself. I have a spare throttle body but honestly I don't want to remove them and find out because it's a royal pain to get everything set properly again.

 

Went ahead and adjusted the TPS properly. Still dying on cold start. Not sure where it could be coming from. Maybe an issue withe the AWS?

 

Sounds like a vacuum leak to me.

 

So, I found something interesting. As I said, the AWS would rev the car to 3k for about 1-2 seconds, then comes all the way down and almost dies.

At the beginning of this thread WondrousBread had said that the ECU really didn't care if the coolant sensor on the bottom of the radiator (with the 2 pins) was connected or not.
Out of pure curiosity, I plugged in that sensor and now the AWS holds the idle like it should.

Only issue now is that it holds it at about 2750 instead of 3k, comes down to about 800, then holds about 1200. While it's holding at 1200, it'll bounce between that and 1500.

 

Right, so that's what I meant. The AWS is only there for emissions reasons. I have that switch unplugged, the car starts and revs up to ~1.5k, then slowly goes down to a solid 900rpm when warmed up. It should be 750rpm, but that's for other reasons. This 17 second warmup is to light off the precats quickly for e-test reasons.

If I plug the switch in it does exactly what yours does for 17 seconds (~3000rpm, 2750rpm is not unusual) then resumes it's normal routine.

If you like the 3000rpm high idle then you can leave it plugged in and it won't hurt anything, but your other problems are unrelated. That switch literally does only one idle related thing; the 17 second high idle. Otherwise it has one other switching function for the ACV that AFAIK does not affect idle.

 

Right... understood. So I guess that leaves me with a few things here that I still have questions about. Once it's hot, it idles just fine, which is making me think more and more that it could be thermowax. But, once I get on it, the acceleration isn't as linear as you'd expect. There are spots in the rev range where it'll stop accelerating for a moment, and then go back to it. Not sure if this is anyway related to the idle or if it's a separate issue.

1. Could improper fueling (ie, clogged filter, clogged injector, etc) cause what I'm experiencing?

2. Could timing affect either of these?

3. Recently had to replace the leading plug wires after it wouldn't start at all. Didn't replace the trailing. Could the trailing plugs not getting spark also cause this?

Appreciate everyone's patience. Just want to make sure I'm not ruling anything out.

 

So I've made another discovery. Not sure if it's an issue or not, but it's interesting nonetheless.

found this in the FSM that shows thermowax for the MT and AT cars. Thought something looked odd, so I ran out to my car to check it out.

This is what I have:

My car is a 5 speed, yet it looks like it has the thermowax for an automatic?

 

Yep I would swap out that thermowax for a manual tranny one just to eliminate a variable.

Now that you plugged that coolant sensor back in you should readjust the TPS and throttle screws.

 

Got it. Will do that and see if that fixes the issue.

The throttle screws confuse the living hell out of me. I know about the one that directly affects the TPS on the front of the throttle body (facing the front of the car). I know about the throttle stop screw with a jam nut right on top towards the front (not supposed to be adjusted). There's one directly on top of the throttle body in the center. But, from what I hear, there are 2 more. There's one on the BAC, and there's one that adjusts the fast idle cam for the thermowax? Any explanation on which one's do exactly what, and which one's I actually need to adjust would be much appreciated.

TPS is currently set properly. Reads 1.08 closed throttle and 4.9 WOT.

 

Adjust the TPS to 1.00 closed throttle then adjust the set screws as per FSM.

 

Not trying to hijack your thread. But I think I have the same issue with my vehicle. The switch wires broke on my radiator temperature sensor switch. I haven't run the vehicle more than 10 minutes since. Even though it's summer it now won't hold idle. After starting just fine. I do want that switch replaced because I live in Denver and I'd like to drive it in the winter.
Do you happen to have a source for that replacement part?
Thanks.