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Author Topic: SOLVED?? Misfiring or fuel cuts out under load?  (Read 1335 times)

jdp

  • Jr. Member
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  • Posts: 68
    • 1994 Magenta Capri NA, 94 Reef greenXR2 and 92 XR2
Re: Misfiring or fuel cuts out under load? - NOT BOOST ALARM
« Reply #15 on: March 10, 2018, 12:05:55 PM »

I did a quick google search for Capri TPS and only found any at Modern Capri....$300 for a new one and $148 for used.   Not sure if a mazda TPS will crossover.  I did look at 88 mazda 323 and the picture looked different.  Maybe someone will chime in if they know.
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John

boomingbeetle

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    • '92 XR2, '71 Porsche 914, '79 Trans Am, 13 Outback
Re: Misfiring or fuel cuts out under load? - NOT BOOST ALARM
« Reply #16 on: March 10, 2018, 12:43:29 PM »

Ha, now I am second guessing myself! I only found the graphs online but this morning I found the whole manual on tech Capri. The TPS diagnosis procedure is very confusing to me in the manual, but there is also a voltage check and voltage graph too. Itís raining heavy today, so I canít try this one but Iíll give it a go tomorrow.
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boomingbeetle

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    • '92 XR2, '71 Porsche 914, '79 Trans Am, 13 Outback
Re: Misfiring or fuel cuts out under load? - NOT BOOST ALARM
« Reply #17 on: March 10, 2018, 12:46:25 PM »

Also, Mike h is parting out a car. I bought his TPS and a few other parts. The NA and turbo have the same tps. I will compare the numbers next week when the unit arrives. I might actually get something done after work since daylight savings, it wonít be pitch dark when I get home. I need a garage... thanks again Mike!
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SHOwn

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    • 91 XR2
Re: Misfiring or fuel cuts out under load? - NOT BOOST ALARM
« Reply #18 on: March 10, 2018, 03:52:18 PM »

Let me know if they are the same, I may have one from a 94 N/A I stripped.
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R. Powell
'91 XR2, LM 16" Chrome Drifters, White Gauge Faces, Kuhmo 40's, Big Brake UG, 2.5" Mandrel Bent CATback, BMW318 Heated Seats, `94 console, '99 Dodge Neon Spoiler, '94 XR2 Sideskirts Bumpers and taillights, Fresh Basecoat-Clearcoat Cardinal Red (Jul2014), vj11/vj14 hybrid, manual boost CTL.

chrispoe

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    • 91 BP Capri GT
Re: Misfiring or fuel cuts out under load? - NOT BOOST ALARM
« Reply #19 on: March 12, 2018, 09:46:06 AM »

The procedure for the Throttle Position Sensor (TPS) is similar, but we are testing for resistance ohms instead of voltage.  You don't need the car on for this one, just unplug the sensor and probe the TPS pin and the signal pin.  The reading graph looks like a bell curve, when throttle is 1/8 you should read about 1,000 ohms and peak to near 1,500 ohms around 1/2 throttle, then back down near 1,100 at full throttle.


The resistance should look like this.



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boomingbeetle

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    • '92 XR2, '71 Porsche 914, '79 Trans Am, 13 Outback
Re: Misfiring or fuel cuts out under load? - NOT BOOST ALARM
« Reply #20 on: March 12, 2018, 10:27:12 AM »

Well, bad news.  I re-tested the TPS with the harness connected and powered, and both my voltage and resistance readings were close to the service manual charts.  I will post pictures of this later when I have a chance.  I did fiddle with the TPS anyway, but my symptoms persist.  Rotating the TPS maximum CCW resulted in the check engine light being on at idle, but it would go off as soon as I touched the gas.  No difference in performance at any other setting.  I will still try the used unit I should receive this week and compare.

Additionally, I swapped out the fuel pump for a new one that came with the car.  I wish I had taken pictures of this procedure for record but I completely spaced it.  I also put new spark plug wires just because, I even added dielectric grease around the ceramic insulation of the plug as an added layer of protection against shorting.  Car still stumbles.  Next step is O2 sensor, then the ECU/computer I guess.
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SHOwn

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    • 91 XR2
Re: Misfiring or fuel cuts out under load? - NOT BOOST ALARM
« Reply #21 on: March 12, 2018, 06:03:48 PM »

Man, I hate when that happens. Ya think you have it figured out and it squirms out of your grasp.
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R. Powell
'91 XR2, LM 16" Chrome Drifters, White Gauge Faces, Kuhmo 40's, Big Brake UG, 2.5" Mandrel Bent CATback, BMW318 Heated Seats, `94 console, '99 Dodge Neon Spoiler, '94 XR2 Sideskirts Bumpers and taillights, Fresh Basecoat-Clearcoat Cardinal Red (Jul2014), vj11/vj14 hybrid, manual boost CTL.

boomingbeetle

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    • '92 XR2, '71 Porsche 914, '79 Trans Am, 13 Outback
Re: SOLVED?? Misfiring or fuel cuts out under load?
« Reply #22 on: March 15, 2018, 11:01:12 PM »

Well... if itís stupid, and it works, it ainít stupid! 

So I decided to try taking the whole grounding bracket off of the thermostat housing (the thing that the grounds bolt to and the O2 sensor harness clips onto).  I only got the bolt about half a turn out and coolant started to seep out of the top of the housing, so I wiggled the bracket and retightened the housing. Then I connected the grounds to the negative battery terminal with an alligator clip. NO STUMBLE / MISFIRE / HESITATING / POWER LOSS for two days of driving. I donít understand it, but Iím happy about it. I took the alligator clip off and it still works, so maybe I scraped through some oxidation or helped the connection by messing with the t-stat bolts. I might try just un-grounding those wires to see if it starts acting up again, but honestly Iím too afraid I wonít be able to reverse it!  I promise Iíll post some more pics including TPS diagnosis this weekend.
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SHOwn

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    • 91 XR2
Re: SOLVED?? Misfiring or fuel cuts out under load?
« Reply #23 on: March 16, 2018, 05:20:36 AM »

Man, I hope it stays good. If so, your persistence paid off. If it comes back you might look for other grounding locations to try or maybe replace the thermostat and gasket- maybe the housing as well. Iím just wondering that if it is that touchy, why should they have ever used that bolt in the first place. IIRC, the darn thing gets sealed in place so you donít have a leak through the bolt hole like you experienced. How is that actually supposed to ground well, through the coolant?
Anyway, that will be great if it keeps purring.
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R. Powell
'91 XR2, LM 16" Chrome Drifters, White Gauge Faces, Kuhmo 40's, Big Brake UG, 2.5" Mandrel Bent CATback, BMW318 Heated Seats, `94 console, '99 Dodge Neon Spoiler, '94 XR2 Sideskirts Bumpers and taillights, Fresh Basecoat-Clearcoat Cardinal Red (Jul2014), vj11/vj14 hybrid, manual boost CTL.

boomingbeetle

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    • '92 XR2, '71 Porsche 914, '79 Trans Am, 13 Outback
Re: SOLVED?? Misfiring or fuel cuts out under load?
« Reply #24 on: March 16, 2018, 10:16:54 AM »

IIRC, the darn thing gets sealed in place so you donít have a leak through the bolt hole like you experienced.

It was actually leaking at the t-stat gasket seal at the block, not the bolt hole. Even with half a turn on the upper bolt, the thermostat housing started to separate from the block. And Iím sure part of the problem with the ground is corrosion of the bolt threads because that sucker is TIGHT
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chrispoe

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    • 91 BP Capri GT
Re: SOLVED?? Misfiring or fuel cuts out under load?
« Reply #25 on: March 16, 2018, 04:43:53 PM »

Seems logical to me

You said the engine ran fine when first started (below operating temp) or when you unplugged the TPS sensor and created an operating fault which forced the ECU to run back in open loop mode. In this mode the ECU pretty much determines the amount of fuel to provide by the barometric pressure sensor, coolant temp sensor, Vaf(flapper door and air temp), and the RPM.

When the engine gets to operating temp and there're no major faults, it goes into closed loop mode. In this mode and the engine running at a steady RPM, the ECU will use the feedback of the O2 sensor to help adjust the fuel.

Those ground wires that connect at the thermostat housing run directly back to the ECU to provide the ECU with an engine ground reference point.  This is important for the proper operation of the ECU to determine the true state of the O2 sensor. The O2 sensor only has a range of 0-1 volt, if it's below .5V the ECU read it as lean, above and it's rich.

With the engine reference ground disconnected, the ECU ground started to float a little and created a bias that made the ECU interpret the O2 sensor's signal as being higher than it really was.
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SHOwn

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    • 91 XR2
Re: SOLVED?? Misfiring or fuel cuts out under load?
« Reply #26 on: March 16, 2018, 06:18:13 PM »

See, that kind of analysis is priceless to those of us who just stumble along. It would take me days to begin hunting down that path. Thanks for documenting that in this thread. It helps to know where to start when you have a symptom. All I knew about grounding was what I read in an article in the RockAuto newsletter Tom Taylor wrote a few months ago.
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R. Powell
'91 XR2, LM 16" Chrome Drifters, White Gauge Faces, Kuhmo 40's, Big Brake UG, 2.5" Mandrel Bent CATback, BMW318 Heated Seats, `94 console, '99 Dodge Neon Spoiler, '94 XR2 Sideskirts Bumpers and taillights, Fresh Basecoat-Clearcoat Cardinal Red (Jul2014), vj11/vj14 hybrid, manual boost CTL.

jdp

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    • 1994 Magenta Capri NA, 94 Reef greenXR2 and 92 XR2
Re: SOLVED?? Misfiring or fuel cuts out under load?
« Reply #27 on: March 16, 2018, 09:03:29 PM »

I just changed the thermostat and coolant temp sensor for the fan today in my 94 turbo.  In doing so, I too realize that the stud the ground wires are bolted to are twice removed from the block via the cruise cable bracket and thermostat housing.  I would not have thought it necessary to pull the bolt on the thermostat housing to look for corrosion, I would have just cleaned the stud the ground wires were bolted.  my 92 is having the same stumbling issues that Boomingbeetle was having....I might have to checkout that thermostat housing bolt too on that car.   Thanks Boomingbeetle for keeping us updated on the final outcome.
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John
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