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Tech, Repairs, Upgrades => Capri XR-2 89-94 => Topic started by: boomingbeetle on March 05, 2018, 12:04:42 PM

Title: SOLVED?? Misfiring or fuel cuts out under load?
Post by: boomingbeetle 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!
Title: Re: Misfiring or fuel cuts out under load? - NOT BOOST ALARM
Post by: boomingbeetle 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 “safe mode”, but unplugging the TPS doesn’t mean that the TPS is necessarily faulty? It could be any other sensor/ECU that is faulty and I’ve 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?
Title: Re: Misfiring or fuel cuts out under load? - NOT BOOST ALARM
Post by: SHOwn 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.
Title: Re: Misfiring or fuel cuts out under load? - NOT BOOST ALARM
Post by: boomingbeetle 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.
Title: Re: Misfiring or fuel cuts out under load? - NOT BOOST ALARM
Post by: SHOwn 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.
Title: Re: Misfiring or fuel cuts out under load? - NOT BOOST ALARM
Post by: jdp 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.
Title: Re: Misfiring or fuel cuts out under load? - NOT BOOST ALARM
Post by: boomingbeetle on March 09, 2018, 11:00:00 PM
Well, I started diagnosing things and making adjustments again.

Spark Plugs look Great:

(http://i63.tinypic.com/24llrav.jpg)

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

(http://i67.tinypic.com/2cf6003.jpg)

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

(http://i63.tinypic.com/azb1uv.jpg)
Title: Re: Misfiring or fuel cuts out under load? - NOT BOOST ALARM
Post by: boomingbeetle 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)

(http://i68.tinypic.com/33kd3k5.jpg)

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

(http://i65.tinypic.com/2qjlzjm.jpg)

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

(http://i65.tinypic.com/2zhnof8.jpg)

(http://i65.tinypic.com/2jdm746.jpg)

(http://i64.tinypic.com/29pcm1l.jpg)

The rear engine grounding strap at the body:

(http://i65.tinypic.com/2mmxues.jpg)

(http://i66.tinypic.com/1z5j1o2.jpg)
Title: Re: Misfiring or fuel cuts out under load? - NOT BOOST ALARM
Post by: boomingbeetle on March 09, 2018, 11:20:22 PM
Still No luck, so I went to the fuel pump and tried cleaning that ground too. 

(http://i65.tinypic.com/2qmeloo.jpg)

(http://i66.tinypic.com/4tlkk7.jpg)

(http://i68.tinypic.com/2hz61pf.jpg)

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...

Title: Re: Misfiring or fuel cuts out under load? - NOT BOOST ALARM
Post by: boomingbeetle 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

(http://i64.tinypic.com/2lmuyh1.jpg)

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

(http://i66.tinypic.com/jv6w5u.jpg)

And full open I am at 7.6V

(http://i68.tinypic.com/xemxxs.jpg)

Looks like the VAF is good.  On to the TPS...
Title: Re: Misfiring or fuel cuts out under load? - NOT BOOST ALARM
Post by: boomingbeetle 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

(http://i65.tinypic.com/2zqrnyv.jpg)

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

(http://i64.tinypic.com/2hefqjd.jpg)

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

(http://i68.tinypic.com/o1jkn.jpg)

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

(http://i65.tinypic.com/s3k9so.jpg)

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...
Title: Re: Misfiring or fuel cuts out under load? - NOT BOOST ALARM
Post by: boomingbeetle 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.
Title: Re: Misfiring or fuel cuts out under load? - NOT BOOST ALARM
Post by: greywolf27030 on March 10, 2018, 09:10:52 AM
I think if I need a mechanic, I'm calling you.

Jack Byrd
Title: Re: Misfiring or fuel cuts out under load? - NOT BOOST ALARM
Post by: SHOwn on March 10, 2018, 10:32:04 AM
Wow wow wow! Great write up! Haven’t 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 can’t remember. Thanks so much for taking the time to document the troubleshooting as you worked. Not enough of us do that.
Title: Re: Misfiring or fuel cuts out under load? - NOT BOOST ALARM
Post by: jdp 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.
Title: Re: Misfiring or fuel cuts out under load? - NOT BOOST ALARM
Post by: jdp 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.
Title: Re: Misfiring or fuel cuts out under load? - NOT BOOST ALARM
Post by: boomingbeetle 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. It’s raining heavy today, so I can’t try this one but I’ll give it a go tomorrow.
Title: Re: Misfiring or fuel cuts out under load? - NOT BOOST ALARM
Post by: boomingbeetle 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 won’t be pitch dark when I get home. I need a garage... thanks again Mike!
Title: Re: Misfiring or fuel cuts out under load? - NOT BOOST ALARM
Post by: SHOwn 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.
Title: Re: Misfiring or fuel cuts out under load? - NOT BOOST ALARM
Post by: chrispoe 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.
(https://imgur.com/HJuLvJX.png)


Title: Re: Misfiring or fuel cuts out under load? - NOT BOOST ALARM
Post by: boomingbeetle 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.
Title: Re: Misfiring or fuel cuts out under load? - NOT BOOST ALARM
Post by: SHOwn 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.
Title: Re: SOLVED?? Misfiring or fuel cuts out under load?
Post by: boomingbeetle on March 15, 2018, 11:01:12 PM
Well... if it’s stupid, and it works, it ain’t 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 don’t understand it, but I’m 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 I’m too afraid I won’t be able to reverse it!  I promise I’ll post some more pics including TPS diagnosis this weekend.
Title: Re: SOLVED?? Misfiring or fuel cuts out under load?
Post by: SHOwn 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. I’m 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 don’t 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.
Title: Re: SOLVED?? Misfiring or fuel cuts out under load?
Post by: boomingbeetle on March 16, 2018, 10:16:54 AM
IIRC, the darn thing gets sealed in place so you don’t 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 I’m sure part of the problem with the ground is corrosion of the bolt threads because that sucker is TIGHT
Title: Re: SOLVED?? Misfiring or fuel cuts out under load?
Post by: chrispoe 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.
Title: Re: SOLVED?? Misfiring or fuel cuts out under load?
Post by: SHOwn 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.
Title: Re: SOLVED?? Misfiring or fuel cuts out under load?
Post by: jdp 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.
Title: Re: SOLVED?? Misfiring or fuel cuts out under load?
Post by: HarryN on April 02, 2019, 05:09:53 PM
Thanks for posting the diagnostic information in this thread.

My son's car is having some kind of issue with cutting out at 2 - 3K rpm and I will use some of your methods to look at options.
Title: Re: SOLVED?? Misfiring or fuel cuts out under load?
Post by: HarryN on April 08, 2019, 02:21:15 PM
Following your advice / guidance in this thread, I have been taking all of the ground wire / connections apart.  Cleaning everything up and replacing most of the ground wires with heavier awg mil spec wire / crimps.

You might ask why I started with this approach?  My  son and I build auxiliary power systems for conversion vans so we are already pretty well set up for wire and ring lugs.  We use mil spec wire because it combines the benefits of marine wire (individually tin plated fine strands) and extended temperature range (down to (-55 C) )    Normal marine wire is only made for boating type weather conditions.

90% of the ground connections looked "ok" but a few are marginal and at least one had a thread locker on it which might have been making it a dicey connection.

 Also going to add a few direct connections back to the negative battery terminal.


Title: Re: SOLVED?? Misfiring or fuel cuts out under load?
Post by: JJ on July 09, 2019, 12:53:54 PM
Well... if it’s stupid, and it works, it ain’t 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 don’t understand it, but I’m 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 I’m too afraid I won’t be able to reverse it!  I promise I’ll post some more pics including TPS diagnosis this weekend.

glad you got it fixed. I've been away from the boards for a few years. but I had this exact problem for years, and believe I posted that's basically what fixed mine. it went away when i had the radiator/thermostat housing changed.