I have a '90 convertible. I took it out last night, and all was well for about 10 minutes, until I got a check-engine light. Nothing obviously wrong on the gauges.

After a few minutes it started getting cranky. Wouldn't take throttle; anything more than about 1/3 throttle started it missing and bucking. It never outright quit, but it was not happy. I limped home in first and second. After about another 15 minutes, it started running better, would take more throttle without choking, but it still wasn't completely happy.

This morning I started it up. Cranked right up, revved up fine, no check-engine light, apparently back to normal. I hate intermittent failures

I used the test port to get out ECU codes. If I'm reading it right, it's complaining of codes 20 and 27, which are both related to the MOP (Metering Oil Pump) / Fuel Injection system. This makes some amount of sense to me, it acted like a fuel delivery problem.

I'm looking for suggestions on how to proceed. Has anybody seen this kind of problem? I'm going to clear the codes and do a bit of basic maintenance. I'm also going to dig through the shop manual and see if anything jumps out at me. If nothing obvious jumps out at me, am I good to keep running it? Or if I have an MOP problem, is that a time bomb waiting for me?

Thanks in advance for any hints.

 

you're on the right track, clear the codes, and see if it comes back. when it does, step 2 is to check the Metering pump wires and connectors, if you're lucky its a bad/dirty connection. if that doesn't fix it, you need a new/new to you Metering pump

 

Well, dunno.

I cleared the codes, cleaned up a few things, changed oil because it was almost time, and looked around the engine bay. It's 31 years old, but aside from that looked all ok. I peered around with a flashlight, trying to see if there was any evidence of leakage from the oil lines, but it all looked dry. Buttoned it up, drove it around a while, no issues, it was running as strong as ever.

I guess I'll just have to keep driving it and see what happens next.

I may try to get at that MOP connector block. It's a little buried down there, but it seems like a good move to check/clean it.

 

the Metering Pump essentially works the same as a drive by wire throttle on a new car, there is a stepper motor and a position sensor. for a given RPM and load, the metering pump should be in a certain place, and if its not, then power gets restricted to the point where you don't damage the engine.

 

Ah, ok, I didn't realize it was controllable. as well as readable, by the ECU. Thus your comment about the connector. That makes sense.

I haven't yet had time to go in there and get at that connector. I have started it up and run it around a couple of times, and it's been fine. It sounds like I'm relatively safe to run it some, as if it acts up again, the ECU will put me back in limp mode.

Thanks!

 

Well, it did it again. After running for 20 minutes, CEL. Got home, checked codes

• 17 - Feedback System (poss O2 sensor) - Feedback cancelled
• 20 - Metering Oil Pump (MOP) position sensor - Basic Fuel Inj and fixed timing
• 27 - MOP (General) - Basic Fuel Injection and fixed timing

I'm not sure what to make of 17, or why that would be connected to the MOP. For 20 and 27, I've now got to pull a few things apart. First get to that connector, open it up and clean it. If that doesn't do it, I'll need to start looking for a replacement MOP. Any recommended sources for parts like that? ebay doesn't seem to have a lot...

 

. . . and, a little more digging around in there suggests that the only way to get the connector off is to pull the MOP off the engine. And in order to do that, I'll need to pull the air pump and lower bracket out.

I'll also need to remove the 4 oil lines that go from the MOP up to the rotor housings. The book says that there are o-rings in there which need to be replaced. So it sounds like I ought to be ordering some parts before I tear into this.

Am I missing something? Is there an easier way to get at that connector?

Thanks in advance for any wisdom here.

 

Next: I got a replacement MOP. I've gotten the smog pump out of the way, and removed (more or less) the four oil lines from the MOP up to the rotor housings. Ready to swap the new MOP on.

I'm leaking oil out of the oil lines. Do they need to be purged before I run the engine again? My understanding is that the volume of oil is small, so it may take a while for these bubbles to run up to the rotors. If I'm making a problem for myself, I'd like to sort it out now before I start driving the thing again.

TIA for any hints.

 

Well, rats.

I changed out the MOP. (what a pain! it's just about impossible to get at) I had plugged the oil lines, pretty sure I didn't introduce much air into the system. Buttoned everything back up. Drove it around, and 10 minutes later, CEL again

Now the codes are saying 17, 27, and 51. 17 is O2 sensor. 27 is the MOP. 51 is "Fuel pump resistor relay". Not sure what to think about 51, the others are old favorites.

Would a busted O2 sensor cause the ECU to go into limp mode? 89-91 CPU Trouble Codes makes it sound like yes. But even if I address that, it sounds like the MOP is still unhappy. It's also possible that I have a wiring problem. Is there anything special I should look for? Other than just tracing the wires from the MOP connectors back to the ECU?

As always, thanks in advance for any wisdom.

 

Epilogue: I've ended up deciding that it was the connectors after all.

I did change out the MOP, and cleaned the connectors, as above. But it kept flaking out. Because it's easier to clean the connectors than go after the MOP again, I went through several more rounds of cleaning. I made a very narrow wood tool to get contact cleaner in there. After a dozen wipes with that on each contact, I stopped getting dirt out. That got me more and more run time, but it would still throw a CEL after a while. I ended up reaching into the male connector blocks and giving a very very slight twist to each of the fingers, hopes it would do a better job contacting the female side.

That seems to have done the trick. I've been driving the car, no more CEL. My surmise is that the spring contacts in the female side of the connector have lost some of their springiness. What with vibration, and even a tiny amount of corrosion in there, the connector goes intermittent. That's the best explanation I have so far. Just thought y'all would want to know.

 

Well: Last year I had this sorted out. The car ran great until I put it away for the winter.

This spring, it's at it again. I've cleaned the connectors multiple times, but no go. I've tried adding a little more twist on the male connectors, no go. It does seem to take a little longer each time for the CEL to pop on and the ECU to start starving me for fuel, but I can't get it to last more than about 10 minutes. Maybe if I just keep driving it . . .

I want to replace these verdammt connectors. I could just get some kind of 3 and 6 pin connector bodies and graft them in, but that bugs me. Does anybody have leads where I might be able to get Mazda OEM connectors from the wiring harness to the MOP?

TIA . . .

 

Haven't posted in a while, mostly because I've been too busy to work on this thing more than 20 minutes at a time.

I have thought multiple times that I was on the track, of fixing my check-engine-goes-into-limp-mode issue, but it's not working.

I've been all through the MOP, including changing it out, cleaning and changing the connectors, basically digging into everything I could find. It still runs for a while, then flakes out on me.

I talked to my brother, who's done a lot of car hacking on Mazdas, including racing them. He believes I'm on the wrong track. What if there's something other than the MOP causing the ECU to lose its lunch? I've been through the MOP and related stuff, but I haven't really looked hard at anything else. So my question for the group: What other kinds of issues can cause the ECU to go into limp mode? Faulty TPS? Air volume sensor? It's a 1990, so it can't be too complicated, by modern standards.

I also have scoured through the shop manual, looking for some hints about what the ECU is looking for. That manual isn't very well organized, IMHO, so I may have missed it. If anybody knows of a secret place where that's documented, that would be much appreciated. Or if there are shops with folks who know these aging beauties, that would be appreciated as well.

 

I had very similar issues. My omp actually fried my ecu. Pull the ecu and take it apart and see if anything looks burnt in there.

this was mine. See the burnt part in the middle? the big rectangle deal towards the bottom left was burnt too. cant see it in the pic though

 

Wow. Hadn't considered that kind of ECU failure. Were you getting symptoms like going into limp mode with no obvious reason?

 

exactly. I could cruise and it would go into limp mode after a while. Or if i tried to rev it too high, it would go into limp mode. I traced it back to a broken wire on the omp, which must have then fried the ecu. Replaced both, and runs just fine now.

 

Omp ?

 

OMP? MOP? Do you remember which wire?

 

yeah, MOP, same thing. Nah, i dont remember which wire. Id still check the ecu. at least for peace of mind.

 

Sounds like a good idea. Time to pull up that carpet . . .

 

Got a couple more pics of my burnt one. It actually burnt the case of the ecu.