Need a Miata Guru! | 2200RPM Limp Mode??

Hey all, looking for some Miata advice. 

Got a 94 shell/body that I put the good running motor from my 95 into. Having a few issues. 

The main issue is that when it first starts up it's fine and revs ok, but then after 20 seconds or so it won't rev past 2200rpms. 

Known issues:

Vacuum Leak: (Resolved) There was a vacuum leak, on the intake manifold, one of the nipples that gets plugged. I plugged it up and now it idles better, the RPM issue still persists. 

Cam Sensor: In the process of putting the motor in, I damaged the (intake) cam sensor. I pulled the sensor on my spare engine and replaced it. It seemed plug and play, unless there is some process to installing them properly that I am totally unaware of. 

Oil Pressure Sender: First time I started it up I noticed no oil pressure, according to the gauge. Verified it did in fact have oil, and I hadn't changed anything else since pulling the motor that would affect oil pressure (ran fine previously) so I figured I just missed plugging it in that sensor. I was able to drive around for a while, then the RPM issue came up. Looking into it, 94 was the only model year with the 1.8l engine that had the old (real) oil pressure sender, and 95 and newer had a essentially a dummy light sender. After plugging in the wire to the 95 sender, I get no reading still.

So, is it possible that it is going into some sort of limp mode due to reading 0 oil pressure?

As in, would buying the proper ~$200 90-94 sender likely fix my issue? 

MAF Wiring: When I put the engine into this shell I found that the insulation on the MAF sensor wires was gone, and the black wire (with green stripe I think?) was cut and stripped at both ends, but at the harness side there were two black (with green stripe?) wires cut and stripped. Right now if I put alligator clips between the two at the harness and the one at the MAF, it doesn't start, but if I also ground that connection it starts up fine. Looking at wiring diagrams, if I am actually reading it right, it seems like that wire should be ground… Is that right? Not exactly sure why someone would mess with the wiring there, but maybe the chipmunks got to it at some point??

I did also did an EGR block-off if that could have any impact as well. 

Sorry I don't have anything specific, but I would be looking hard at the cam sensor, and the MAF wiring.  Also check the throttle body connection... that has been known to cause similar issues.

AFAIK, there is no 'limp' mode programmed into the NA ECUs...

Rodan said:

AFAIK, there is no 'limp' mode programmed into the NA ECUs...

That's the main thing I want to confirm. Because if you're right, then I just need to do more diagnosis. 

Seems to be a common problem... found several threads on the forums.  Most point to connector issues, or ignition...

thread

thread

 Will mess with connectors more then. 

I'm not sure exactly how the stock ECU will react to the deleted EGR, but I'd put that back in if I were you.

The ECU does not monitor oil pressure at all, so that's not your problem. I've seen this sort of behavior with an aftermarket ECU with pinned 10 degree timing (as opposed to a proper timing table) - I'm assuming you actually set the base timing? I'd also take a very close look at any wiring fixes you had to make.

In reply to Keith Tanner :

Hmm...

So when the motor was in my old miata, I had the EGR deleted, partially. As in, I ended up cutting the EGR tube to get the exhaust manifold off, capped off the port on the exhaust manifold and then plugged the tube going to the intake instead of taking it off and doing a block-off plate. Well, when I pulled the engine out of my old miata I found that the tube plug didn't really seal very well at all so it was venting to atmosphere really. So while I had the engine out, I fabbed up a block-off plate quick before I put it in. 

Given all that, it sounds like a reasonable test would be to take off the block-off plate and see how it reacts to atmosphere, and then look at getting a new EGR tube. 

"Venting to atmosphere" sounds like a massive vacuum leak to me.

I don't know exactly how a 1994 ECU responds to a malfunctioning EGR system, but if you have all the parts off the parts car I'd put them in. 

In reply to Keith Tanner :

I might just not be understanding it correctly at all, so forgive me if that's the case, but I don't see how exhaust gasses are much different than atmosphere gasses in terms of vacuum leak... Both situations are just sucking in air into the intake are they not?

I have to check and see if my spare motor still has the EGR tube... Otherwise if the issue continues to point in this direction then I know what I need to order or find used. 

And yes, I know my whole EGR delete deal was a bit sketchy, but it is a beater.... 

Ah, you've got the valve still in place. Got it.

The sketchier your fixes on a beater, the more time you spend trying to make that beater work 

In reply to Keith Tanner :

Ahhh, on the same page now...

You're not wrong.... 

Afternoon snow day update:

1. Verified that the issue is ongoing. 

2. Undid my EGR block-off plate. 

3 .Issue still persists, no change. Seems like the EGR is not the (main) issue. 

While it was running and having the 2200rpm "limiter" I tried unplugging the TPS, and it behaved the same as plugged in. Have not checked it with a DMM yet, but I think that's my next step. 

Also on the list to check:

-Coils

-Spark Plug Wires (apparently not long lived)

-Fuel Filter 

Ooo, good thought on the TPS. I know that some aftermarket ECUs will prevent the engine from running over a certain speed when the engine is at idle. This means you need both neutral switches (clutch and trans) and a functioning idle switch on the TPS. I don't know if the stock ECU will do this, but it's quite possible.

Hoping work is canceled tomorrow due to snow, if so hopefully I'll have some updates.

Good thing I have a torpedo heater in the garage!

TPS tested fine. Verified that my spare TPS had the same readings as well. 

I also noticed that it seems to only occur once the car is warmed up. 

So on a cold start it's fine for probably 5 minutes, then the 2200rpm limiter happens, shut it off and turn it back on and it's good for 10-20 seconds before the issue comes back. 

It may not be the TPS, it may be the wiring from the TPS. Check the idle switch signal at the ECU. 

You are getting differences with temperature.    Check the coolant sensors.

Don't know about rev.limit.

FYI, On my stock '97, the idle air must be metered. Ran like you're describing when I put a small air filter on the IACV. Ran fine when I plumbed it back into the intake track.

The idle air must be metered on all cars with an AFM or MAF. FYI.

Good to know. Thanks!

NermalSnert said:

FYI, On my stock '97, the idle air must be metered. Ran like you're describing when I put a small air filter on the IACV. Ran fine when I plumbed it back into the intake track.

So I should clean my IACV?

Just checked all my fuses, the 20A (AD FAN) fuse was blown. Replacing it did nothing. 

I also realized that my headlights stopped working, probably about the same time this issue started, which is why I started checking fuses. "Pop-up button" works, high beams work if you flash them, but turning on the headlights does nothing. Doesn't pop them up, doesn't turn them on. Might be the switch I guess. 

I'm not satisfied with the test I got on the TPS at the ECU plug. I need to find a small diameter wire so I can make sure I get a good contact on the pin.