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1990 Reatta: Vehicle “Bolts” or “Lags” at 3 Second Intervals when Travelling at Highway Speed


BRD
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My 1990 Reatta seems to “bolt,” "studder," or “lag” every three seconds – ever so slightly – when travelling on highway and not pressing on the accelerator.  [This is not the classic transmission “bolt” when accelerating, going up a grade, etc.]  Happens with no significant pressure on accelerator and just “coasting.”  To also be clear, I am not using the cruise control system when this happens.

 

When you are really in tune with the car, it gives the sensation that someone is slightly stepping on the brake (for a millisecond) enough for you to lurch forward (slightly) into your seat belt every three seconds.

 

Not sure how to exactly describe but seems that power is cut to the engine and the car noticeably “hesitates” for a fraction of a second, every three seconds.  Happens when at highway speeds (e.g. 50-80 mph) and around 1800 RPM.  Car drops from 1800 to 1600 RPM for a split second, and then back to solid 1800 for three seconds.

 

This has happened for thousands of miles and days of coast-coast highway driving over the last few years.  Greatly appreciate any insight or advice as to the possible cause!

 

BRD    

Edited by BRD
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The first question that comes to mind to help figure out the problem is - does it do the same thing if you are traveling the same speed without cruise and just hold your foot steady on the accelerator?

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  • BRD changed the title to 1990 Reatta: Vehicle “Bolts” or “Lags” at 3 Second Intervals when Travelling at Highway Speed

Hi Ronnie, thanks for the response.  My original wording might have been confusing, I am not using the cruise control system when this happens and have not used cruise control on my Reatta for years.  So this happens when my foot is resting ever so slightly on the accelerator and driving on the highway - at consistent highway speeds.  I think this also happens when my foot is not applied to the pedal at all and I am just coasting (at 50-80 mph).

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My bad. I didn't take the time to carefully read what you had written and  when I saw "cruising" I was thinking cruise control was on.

 

21 minutes ago, BRD said:

So this happens when my foot is resting ever so slightly on the accelerator and driving on the highway - at consistent highway speeds.  I think this also happens when my foot is not applied to the pedal at all and I am just coasting (at 50-80 mph).

That's a tough one. Anything I say would just be a guess. Odd that occurs consistently about every three seconds. Maybe a cylinder dropping out when it hesitates? If that is the problem taking a look at the spark plugs could give you a hint if one of them looks different from the others.

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It sounds like the time torque converter is locking and unlocking. It is normal for it to unlock at speed if you release the throttle or drop below a particular tps value. The ecm does this on purpose as it is preparing for braking. If you coast down for an extended period, the ecm will cut fueling, called deceleration fuel cutoff which will cause the tach to drop a little and then back on. I would drive in diagnostic mode and watch the torque converter indicator to see if it is on and off during this sensation and also watch the tps for what it is indicating. It is the direct indicator to the ecm what you are doing with the throttle

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Tested my Reatta today on highway in diagnostic mode, as suggested by 2seater.  Unfortunately, when the “studder” happens nothing seemed to change with either the Ed01 (TPS) or ES01 (TCC) – if I have those codes correct.  The values I read were:

 

ED01:

Ed01:  0.40  at idle in park at approximately 1100 RPM

Ed01:  0.82 when revving engine in park to approximately 2400 RPM

Ed01:  0.40 when at “idle RPM” on highway

Ed01:  3.28 at 3400 RPM when accelerating on highway

 

ES01:

ES01:  Mostly 00 with "Off" "High" and "Low" all displayed at same time.  Also frequently only "High" and "Off" would display.  Relatively infrequently, 99 would display.  Photos attached.

 

Have not yet been able to test spark plugs.  I was able to confirm today, upon closer inspection, that this studder definitely also occurs when not applying any acceleration (foot off the pedal).  It also happens at lower speeds (just coasting from 40-60 MPH on the highway) with no acceleration.  In fact, it seems to be more pronounced when decelerating (no gas or brakes applied).

 

Gives a similar sensation to when you are downshifting (too early) in a manual transmission vehicle and the car jerks into higher RPM – but on a much much smaller scale/sensitivity.  Although in the case of my Reatta, the RPMs move down a few hundred (not up as when downshifting in a stick).

 

BRD     

Hi Off.png

High Low Off.png

High Off Low Arrow.png

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I am a little confused. ES01 is the override section for manually activating or deactivating functions. The climate control graphic area has all the indicators that function whenever you are in diagnostic mode without having to go any further. I know they are all tiny indicators but they indicate a function is on when lit, and not active when not lit. In this case, the medium fan speed indicator is the TCC indicator and that does not appear to be locked in any of the photos. When lit, it is locked, not lit indicates unlocked. From the photos, OFF lit indicates in fourth gear, fan speed HIGH indicates closed loop, LOW indicates O2 is rich and that little arrow above OFF indicates EGR is on. 


The rpm drop for a few seconds and then release on coast down, most pronounced in the lower speeds can be DFCO cutting fuel. Emissions related I believe.

 

The TPS looks about right but curious about the idle rpm? Was this before the engine was warmed up? I am guessing it was shortly after starting so commanded rpm was elevated. Does it go down when fully warm?

Edited by 2seater
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2seater:  Thanks for the reply.  I looked through the code manual trying to find anything related to torque converter, as you suggested.  Only thing I could find was ES01 - so that's what I checked.  Is there another code that would be better for me to check for the torque converter or DFCO/emissions?  The low fan speed indicator also came on at times during this test, as was captured in one of the photos (but it was mostly off).  Did not have any current error codes when starting up diagnostic system.   

Reatta Codes.jpg

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Nothing else I know of at this point. Just open diagnostics and the section we are interested in is the ECM status lights at the top of the illustrated page. All you need watch is the "MED" fan speed light to see when the TCC locks and unlocks. The one thing I will say about the built in system is that it isn't instant and indication may lag a split second. It is just the nature of the information loop and the multiple paths it takes to light the light. You should be able to get the TCC to lock and unlock while cruising at say 60mph and just roll out of the throttle and then back in. You should see the MED light go out and then back on, with an RPM change of some noticeable amount. That depends a little bit on the load at present and how much slip there is before lockup. In this respect, the touchscreen cars have much clearer indications and actual instructions and labels right on the screen, rather than watching the little degree symbol as it indicates second gear for example. It is interesting to watch that little cluster of information as it changes and follows the gear changes, a/c, tcc, egr etc... but does take some getting used to. Hope this helps.

 

I don't know of an indicator for the DFCO. You could try watching injector pulse width for a definite drop but I know when I have done so in the past it is hard to get a solid indication. As mentioned, the system has a little lag in it, so teasing out small nuances isn't easy.

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Thanks for the response and extra insight 2seater.  I will continue troubleshooting and post if I can determine what's causing this anomaly.  

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I looked for a bit more information on the DFCO and I found the FSM doesn't really give much detail, other than it is decided by the ECM.?  I knew I had seen references to it in the programming of the PROM but most chip burners mention leaving many of those little detail items alone as it has little to do with what we are interested in when adjusting for performance or to accommodate changes. The screen shot below is taken from the stock chip programming in my TunerProRT program. As can be seen, there are eight different items that may be adjusted, so it isn't just a simple coast down vs time or throttle setting that occurs but has settings for rpm, speed of deceleration, tps percentage, temperature etc.. as can be seen. I hope this helps a bit. Even though my car is presently operating with a chip I burned, I haven't a clue what I would have done to modify or improve this item? By the way, LV8 is load value which is a manufactured number calculated from several sensor inputs which I am not 100% sure of. It is the X axis of the spark timing table in the chip, so it is important but it isn't a sensor value. Those load values below are pretty low as the maximum is 255.

 

IMG_1814[1].JPG

Edited by 2seater
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Thanks for posting the screen of the tuner program. Don't mean to hijack BRD's thread but was wondering, do you adjust the parameters with the slider or is that screen read only?

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1 hour ago, Ronnie said:

Thanks for posting the screen of the tuner program. Don't mean to hijack BRD's thread but was wondering, do you adjust the parameters with the slider or is that screen read only?

I would bet each is adjustable but I am not certain, even if it appears to adjust. Sort of a cop out there but there was one situation that may explain: I wanted to adjust the speedometer accuracy for the tires I am running. I noticed there was a setting, pulses per mile, probably from the vss. I changed the number by a percent and a half but when I tried it, I found the same inaccuracy relative to my gps. When I checked it again, the number was back to the original. I looked further and found there was a display pulse divider setting, which I could adjust, did so, and got the speedo within half mph to gps. This corrected the speedometer but not the odometer ?

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I know quite a bit about computer programming but I know nothing about programming the chips in the ECM. I thought maybe you could move the sliders and drive with the laptop hooked up to see what effect moving the slider would have before burning the chip.  Would be nice if it worked that way. Sort of a "try before you buy" type thing ?

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17 minutes ago, Ronnie said:

I know quite a bit about computer programming but I know nothing about programming the chips in the ECM. I thought maybe you could move the sliders and drive with the laptop hooked up to see what effect moving the slider would have before burning the chip.  Would be nice if it worked that way. Sort of a "try before you buy" type thing ?

It probably could be done that way. I bought an emulator from Moates that plugs into my ECM prom through the zif socket in my modified prom. I did actually drive the car running on the laptop but I just have not dedicated the time to it. I have looked at various holders for a laptop in the car but the Reatta is not very friendly to that. A helper in the passengers seat would work far better?

Edited by 2seater
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2seater - thanks for taking the time to put all of that information together and the screenshot above!  Ronnie, thanks again for the comments and additional insight.  I will take this information and continue to troubleshoot.  Will let you know if anything develops.

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  • 1 year later...

Hi everyone, any update on the stutter problem? I own a 1990 for couple days now and experience the same thing at constant Autobahn speed (around 3000rpm). There is a slight stutter, almost like a soft turbo can not decide to kick in or not. I had a Saab 9-3 convertible with a soft turbo and had a similar experience. As with the 1990 Reatta, I think it needs a tune up, which is scheduled for this Saturday. I just hope its not the tranny. Will post an update. 

 

Second question, not related to this issue: Whats should be illuminated/ in the interiour?

mine has passenger footwell, corner of both doors (white and red), dash, lamp on headliner, reading lights (only if activated with a switch), lights in the sun visior/mirror, glove box, the switch for the headlights and fog light. What did I forget?

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