crimson
crimson New Reader
6/10/11 10:56 a.m.

So we have built a Locost running off a 1990 1.6L miata motor running stock ECU. The car has a problem of bogging when you hit about 3000rpm. I am out of idea's of what it might be. The car seem to run fine when it's cool but acts up once it starts to get warm. I tested the TPS and it seems to have conactiviy where is should at the start and end when opening the butterfly. I tested the 02 sensor and it is putting out the proper millvolts when running. I was thinking maybe it was the temp sensor on the back of the block that was bad but when I hook up an ohmmeter to the sensor it is changing as the heat changes. Could it still be bad?? I dunno.

I am running out of idea's.... anyone have any suggestions?

I hooked a light up to the ECU to the yellow black wire to see if I was getting any codes but do not seem to be getting anything from it.

ransom
ransom Reader
6/10/11 11:13 a.m.

3000 rpm always?

3000 rpm under load?

Fuel pressure, perhaps?

I'm pretty sure it'd still be possible for the temp sensor to be bad and still show some signs of life, but it's not my first bet for a serious 3k rpm bog.

Other possible reasons for temp-sensitivity:

  • weak fuel pressure or other fueling system issues masked by cold-enrichment by the ECU until the car warms up.

  • Fuel pump dying after some use (reaching a bit with that one)

  • Is it temp, or just how long its been running; I.e. could you be picking crud off the bottom of the gas tank and plugging up the sock on the fuel pump, and then having it settle away from there when you shut it off?

I'd be a bit surprised if it's got enough juice to start itself but can't do injection/spark properly, but just for giggles, do you have good voltage at the battery when running, and no dodgy wiring between there and the bits that make it run?

I bet you'll get better and more specific responses, but those are the first things that spring to mind for me

alfadriver
alfadriver SuperDork
6/10/11 11:22 a.m.

When I saw the title, I was wondering if there's magic Miata dust available to sprinkle on cars... but I digress.

Can you measure the O2 sensor when it happens? That should tell you, partially, what the bog is- super rich or lean. Both can cause issues, but normally, it's assumed that lean is the cause.

I assume that there are no intake runners to open or close?

And on ransom's points about fuel- it's possible that part of the fuel delivery system is closing down at the right time.

(oh, the load part he asks about is very important- is it RPM related or high load/rpm related.... make sure you post the answers to those questions)

crimson
crimson New Reader
6/10/11 11:33 a.m.

it's 3000rpm under load. You can rev the motor fine when it's just sitting there.

Tom Heath
Tom Heath Web Manager
6/10/11 11:34 a.m.

Fixed the title.

I wish I had more to add to the conversation...

Keith
Keith SuperDork
6/10/11 11:37 a.m.

Can you push through the 3000 rpm threshold? In other words, is it a momentary thing or will the car just not go any further? Has the car ever run well or is this a recent change?

Which temperature sensor did you check, the two-wire or the single-wire? The former is the one used by the ECU, so it's the important one for things like this.

I have seen problems with clogged fuel pumps/filters on freshly built Locost, so it's definitely worth checking the fuel pressure.

Rupert
Rupert New Reader
6/10/11 11:43 a.m.

Be Aware! If you are talking lean under heavy load, you may be holing a piston or two soon! This doesn't take much time under heavy load in a lean engine. This is never fun!!

I would rather err on the side of engine safety than have to do an overhaul.

Rupert

crimson
crimson New Reader
6/10/11 11:48 a.m.
Keith wrote: Can you push through the 3000 rpm threshold? In other words, is it a momentary thing or will the car just not go any further? Has the car ever run well or is this a recent change? Which temperature sensor did you check, the two-wire or the single-wire? The former is the one used by the ECU, so it's the important one for things like this. I have seen problems with clogged fuel pumps/filters on freshly built Locost, so it's definitely worth checking the fuel pressure.

No I cannot push it past the 3000rpm. We were unable to drive the car we got hte motor from so unsure how it ran. The Locost has always had this issue. I checked the 2 wire temp sensor on the back of the motor. What should the fuel pressure be at?? I have a gauge on the line.

alfadriver
alfadriver SuperDork
6/10/11 11:53 a.m.

In reply to crimson:

being a '90, it should be ~39psi, relative to the manifold pressure. So 37-39psi at WOT. If it's falling off of that, well, there's a possible problem.

(assuming that the early engine was typical EFI at the time, with the rail pressure regulator plummed to the intake.... should be- everyone was at the time)

Keith
Keith SuperDork
6/10/11 12:05 p.m.

It is, yes

Is your fuel tank vented? It's possible you're starting to pull a vacuum in the tank, and that'll start dropping fuel pressure on you as you drive. I've seen the stock (insufficiently) vented caps on JAZ tanks cause this problem.

crimson
crimson New Reader
6/10/11 12:09 p.m.

yes the fuel cell is vented. I just took a video of the problem that I am uploading now. Maybe that will help you see what is happening.

crimson
crimson New Reader
6/10/11 12:32 p.m.
Type Q
Type Q Dork
6/10/11 3:49 p.m.

Hmm... could there be an issue with air meter? I am wondering if the ecu bypasses it when cold.

I had some odd hesitation with my '91. I wasn't sure what was causing it. My friend who is Mazda Mechanic asked me when the fuel filter had last been changed. The answer was that I did n't because I had not changed it since I bought the car. Changing the fuel filter cured the issue.

crimson
crimson New Reader
6/10/11 4:27 p.m.

I tried another air meter and no change.

Keith
Keith SuperDork
6/10/11 5:35 p.m.

What's the fuel pressure?

crimson
crimson New Reader
6/10/11 5:42 p.m.

at idel it's about 32psi

Keith
Keith SuperDork
6/10/11 6:45 p.m.

Pull the vacuum line off the regulator and see what the pressure is. Should be at least 38 psi in that situation. What's the pressure when the problem is occurring?

crimson
crimson New Reader
6/10/11 7:14 p.m.

dunno cannot see the gauge when driving. pressure is around 40psi with the vacuum line off the regulator.

alfadriver
alfadriver SuperDork
6/10/11 8:15 p.m.

In reply to crimson:

I'm thinking what Keith is. See if you can rig it to see the fuel pressure, even if you are a passenger.

Is there anyway you can also get the O2 sensor reading? So IF the pressure is ok- have to trace rich or lean, and at least that will give you one or the other, just before it goes bad.

But I think it's leaning toward a fuel delivery problem.

crimson
crimson New Reader
6/10/11 8:37 p.m.

I changed the filter tonight. See if that helps tomorrow.

crimson
crimson New Reader
6/11/11 6:05 a.m.

I still think it's sensor problem. When the car is cold it works fine but once it starts to get warm it starts having the problem. Open loop vs Cold loop.

AutoXR
AutoXR Reader
6/11/11 8:45 a.m.

Open vs closed.

YOur moms a cold loop

iceracer
iceracer Dork
6/11/11 8:46 a.m.

Often overlooked is fuel volume.

Temperature sensors have specific resistance. 68 degeres F.-- 37'300 ohms for example. 194 " 2'800 " You also can check the voltage at the ECT signal wire. In the above cases, it should be 3.07 and .06 V. Tap on the sensor, wiggle the wires and connector. See if voltage changes, it should not.

alfadriver
alfadriver SuperDork
6/11/11 9:34 a.m.
crimson wrote: I still think it's sensor problem. When the car is cold it works fine but once it starts to get warm it starts having the problem. Open loop vs Cold loop.

Except at wide open and very hard throtte, the O2 sensor isn't used.

It would be really good to know if it's far too rich or far to lean.

You said that you can run hard up through the gears when the engine is cold? What happens if you take the engine temp sensor connector off?

Or what happens if you take out the O2 sensor? (it should still run)

You should be able to go though each sensor and tell if it's a partial cause- each sensor should have a basic back up if you disconnect it, and nothing else.

crimson
crimson New Reader
6/20/11 6:21 p.m.

Looks like I found the problem.... Some of hte foam from the fuel cell made it's way into the pump

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