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Author Topic: SOLVED?? Misfiring or fuel cuts out under load?  (Read 1509 times)

boomingbeetle

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    • '92 XR2, '71 Porsche 914, '79 Trans Am, 13 Outback
SOLVED?? Misfiring or fuel cuts out under load?
« on: March 05, 2018, 12:04:42 PM »

So since I fixed the overboosting issue I thought I better start a new thread.  I've read about a lot of people having this issue, seems to be quite common, but I don't know of any definitive answers:

After a couple of minutes with the car running, it seems to cut out like there is no fuel.  If you've ever run out of gas, you know what this feels like.  This only happens while driving under a slight to moderate load, especially between 2000 and 4000 rpms.  It revs smoothly and freely when not in gear, and has never stalled.  Also, the problem does NOT happen if I floor it and go flat out.  I can take off at wide open throttle and go all out through 3 gears and never a miss, but as soon as I lift the peddle off the floor and hold a steady cruise, it will start cutting and bucking.  Makes a pain in the ass cruising on the highway... any ideas? I will troubleshoot the following items in this order, but the $$ will add up so I'd like to know if anyone else has solved this riddle before I get too far in the red with it:

1. just replaced the fuel filter, no change
2. new fuel pump on order, will install this coming weekend
3. replace plug wires (17k miles old)
4. replace cap and rotor (17k miles old)
5. injectors?
6. VAF?
7. O2 Sensor?
8. ECU?

#UPDATE# if you are having these problems, try cleaning the grounds at the thermostat housing including the bracket, connecting bolts, and the top bolt of the thermostat housing first!
« Last Edit: March 15, 2018, 10:45:09 PM by boomingbeetle »
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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 #1 on: March 05, 2018, 09:25:33 PM »

Update:

I started the car and unplugged the TPS, as others have done, and the car runs perfectly with no issue, as others have experienced. I think that rules out plugs, wires, injectors, or fuel pump. I believe I am now running in ďsafe modeĒ, but unplugging the TPS doesnít mean that the TPS is necessarily faulty? It could be any other sensor/ECU that is faulty and Iíve just over-ridden it by unplugging the TPS? I will break out the multimeter and try testing the Unit this weekend, but could I still have a bad O2 sensor or VAF or other item? Can anyone chime in to tell me what others have found or am I going to keep replying to my own questions?
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SHOwn

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    • 91 XR2
Re: Misfiring or fuel cuts out under load? - NOT BOOST ALARM
« Reply #2 on: March 06, 2018, 12:54:31 PM »

I've noticed that traffic on the Capri sites dips in winter months, probably because a lot of owners park them in cold climates. I have not experienced the issues you have, so I've only read about this problem. Have you tried searching this site for TPS and reviewing all the posts that come up? I did and a preliminary scan looks promising. All I can say is in the absence of experienced people to reply, all we have to fall back on is searching. Actually, it is where we all should start, not saying that you didn't. Sometimes I fail to do that.
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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: Misfiring or fuel cuts out under load? - NOT BOOST ALARM
« Reply #3 on: March 06, 2018, 01:10:44 PM »

Yes, lots of searching on this site and other boards... but it just leads to more questions  :o

I'm not frustrated, so I hope it doesn't come across that way.  It seems to me that this is a common enough issue that someone should have pinpointed it, but I don't think anyone has had a definite closure.  It seems odd to me that the same problem could be caused by all of these separate items:

person 1 - bad ECU
person 2 - bad VAF
person 3 - bad TPS
person 4 - bad plug wires
person 5 - corroded wiring harness plugs near and/or involved with brake master cylinder swap
person 6 - corroded wiring harness plugs at ECU due to leaking heater core
person 7 - faulty engine ground
person 8 - vacuum leaks

Most people don't complete the thread with a "SOLVED" update, so I don't really know what has worked, and in many cases the threads are many years old and these people might have moved on from Capris long ago.  Hopefully I will be able to narrow it down and document it, and I will post in other threads as well if I do find a solution.
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SHOwn

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    • 91 XR2
Re: Misfiring or fuel cuts out under load? - NOT BOOST ALARM
« Reply #4 on: March 06, 2018, 02:26:42 PM »

Thanks. Very good synopsis. I agree, most of us fail to report if an issue is solved, we just resume driving and forget to report. Many old posters have moved on. Most of these cars are no longer daily drivers like mine, so they sit a lot and many are just rarely used.
Hope you can find the smoking gun out of all those possibilities.
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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: Misfiring or fuel cuts out under load? - NOT BOOST ALARM
« Reply #5 on: March 06, 2018, 06:10:54 PM »

Boomingbeetle...please do report on what you find.  I just bought a 92 XR2 and driving it today for the first time today to run it through the paces so that I can really start assessing everything thoroughly and putting together my "fix it" list.  Mine was acting just like you describe under load, so I am interested in what you find.  In fairness, though, the car has been sitting for about a 1 1/2 years, so I expect some hiccups as I bring it back to life.   I have only freshen the gas and changed the oil so far, so plugs, wires, cap and rotor are due up next.  I had encountered the arcing from the breakdown of the plug wire insulator down in the head in my wife's 94 NA in the past. That caused a miss under load on her capri, so I am hoping new wires and just plain running this car a little bit will help settle in.
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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 #6 on: March 09, 2018, 11:00:00 PM »

Well, I started diagnosing things and making adjustments again.

Spark Plugs look Great:



Turbo looks great and has no shaft play, spins very smoothly:



The inlet Pipe was disintegrating so I am reconfiguring it with a cone filter.  I'll fab that up later

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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 #7 on: March 09, 2018, 11:12:14 PM »

I wanted to start by cleaning up grounding locations first.  Unfortunately, this did not fix the problem.

Here are the ground connections at the thermostat housing (before)



And the T-stat housing after a wire-wheel and sandpaper:



The Front engine ground at the block and at the body:







The rear engine grounding strap at the body:




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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 #8 on: March 09, 2018, 11:20:22 PM »

Still No luck, so I went to the fuel pump and tried cleaning that ground too. 







None of this made a difference in how the car runs.  I do have a new fuel pump to try, but I'm not convinced that the pump is bad.  I loaned out my pressure gauge so I need to hook it up and check fuel pressure before i swap pumps.  I already replaced the fuel filter.  Time to break out the multimeter...

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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 #9 on: March 09, 2018, 11:34:16 PM »

Testing the VAF (Vane Air Flow) meter.  Sometimes this is refered to as the MAF or MAS for mass air flow but that is not technically correct on these cars.

I had to pull off the VAF box from the air filter in order to reach the vane element (the little air door that meters the intake).  Then I needed to pull back the rubber boot that goes over the connector enough to expose the wires and pins.  The emissions service manual has a basic outline of the procedure but I don't have a breakout box.  Basically, with the key on you use the multimeter in 12VDC and check voltage between the VAF wire and the signal wire.  The manual shows the graph and how the meter should read between 1/8 open to full open.  1/8 = 3.24V and full open = 7.87V plus or minus 15%.

Closed, I get 2.6V



1/2 open I'm at 5.7V which is very close to spec



And full open I am at 7.6V



Looks like the VAF is good.  On to the TPS...
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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 #10 on: March 09, 2018, 11:49:01 PM »

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.

hmmm.  I'm at 526 with throttle closed



And jump up to almost 1900 ohms at 1/4 throttle! not good.



Somewhere around 1/2 throttle I was over 2,000 ohms.  I had to switch scale on my multimeter to k-ohms.  Off the chart high



And it never curved downward.  I max out at 3,700 ohms with wide open throttle.



So pretty safe to say that my TPS is bad.  I won't know if this will solve my hesitation and stumbling problem until I get a new unit installed but I have a pretty good feeling I've found the issue.  until next time...
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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 #11 on: March 09, 2018, 11:52:33 PM »

I think I'll try taking my TPS apart to see if there is a potentiometer inside that I can adjust.  Maybe I can bring it close enough to run the car without stumbling.
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greywolf27030

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Re: Misfiring or fuel cuts out under load? - NOT BOOST ALARM
« Reply #12 on: March 10, 2018, 09:10:52 AM »

I think if I need a mechanic, I'm calling you.

Jack Byrd
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SHOwn

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    • 91 XR2
Re: Misfiring or fuel cuts out under load? - NOT BOOST ALARM
« Reply #13 on: March 10, 2018, 10:32:04 AM »

Wow wow wow! Great write up! Havenít seen anything that good in a long time on this forum. One other stupid little thing I might consider trying would be to spray the thing down with electronics cleaner, or more easily obtained MAF cleaner.  Although maybe that will kill it, which might not be a huge thing since you know yours is bad anyway, it just depends on if you need to drive it for a while. That part might be hard to get affordably, I canít remember. Thanks so much for taking the time to document the troubleshooting as you worked. Not enough of us do that.
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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: Misfiring or fuel cuts out under load? - NOT BOOST ALARM
« Reply #14 on: March 10, 2018, 10:46:48 AM »

Thanks too from me Boomingbeetle!  I am following since my 92 is suffering similar symptoms.  The summery on testing TPS and VAF are great. Hopefully TPS fixes it.
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John
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