Drag racer appears to go lean after it heats up.

Mike Puckett

SCCoA Member
For the last few weeks I've been having a problem where the engine appears to go lean. At the track my time trials are Ok but by the time I start running eliminations my times drop off about 1/2 sec or so. On the way home my A/F gauge which is reading the regular O2 sensors shows that is a bit on the lean side. At idle when this happens my rpm will drop way off and try to stall, I loose the fast idle, and my check engine light will come on until I goose it or pull away from the traffic light. I get a 41 code which is a lean condition. It doesn't matter if my SCT module w/tune is installed or not. We replaced the headgaskets and O2 sensors back in December.
I'm wondering if the O2 sensors could have failed already? I bought 2 new ones to replace them this weekend but I'm wondering if it could be anything else.
Last weekend we had a nice long cooldown befroe the 1st time trial and I ran a 13.18 which is right on the money of what I should have run. My next run was a 13.30 which wasn't unusual due to the heat, humidity, and low barometric pressure. My 1st elimination run was a 13.38 which was also Ok. On my next run it stumbled real bad off the line, bogged and almost stalled and I faded to a 13.80 wasting a .021 reaction time. My 60' time faded from the mid 1.9's to a 2.197. It's done this 2 weekends in a row now. Could those O2 sensors have gone bad so soon? I can monitor either of them and they both read identically. I have no appreciable coolant loss either. Any ideas out there?
 
I can't help you but I am haveing the same probblem. So far I installed a adjustable fuel pressure regulator and gauge, fuel filter, And my 02's have less then 10,000 miles on them. The car runs great untill it get's warm then the AFR starts to go lean and the car idles rough then it will die. I have a wideban 02 meter with it's own 02 sensor. I just drained the gas out tonight and put some new Snowco 94 but I did not get to try it yet. I also cranked the fuel pressure up to 41 from the 37-38 it was stock. I will try it tomarrow and let you know what happens. Let me know if you find anything out.

Paul
 
Have you watched the fuel pressure during these runs? Any chance that the fuel pump is struggling?

Could you be having maf sensor issues?

To test the o2 sensors, can you disconnect them and see how the car behaves? It should go to it's default table and get a tad rich, I believe.
 
Mike8675309 said:
Have you watched the fuel pressure during these runs? Any chance that the fuel pump is struggling?

Could you be having maf sensor issues?

To test the o2 sensors, can you disconnect them and see how the car behaves? It should go to it's default table and get a tad rich, I believe.

No I can't watch the fuel pressure while driveing I just have a rail mount gauge. I am going to look into the MAF tomarrow, And yes I unpluged the 02's and it changed the way the car run.

I have seen this probblem before I think and I think it turned out to be a MAF I just read that in diffrent thread and it reminded me.:eek:
 
When I gun the engine in neutral fuel pressure is varying between 40 and 50 psi. The fuel pressure regulator doesn't get hot plus it's new so I think fuel pressure is Ok. New 255lph hi volume fuel pump. New last December that is. I can't tell if it's really running lean or if the O2 sensors are just saying that. Sometimes they lie! I cleaned the MAF and it looked Ok and I don't get an MAF code, just the O2 sensor code. I'll stick an ohmmeter on it to be sure.
I'm going to pull a couple of plugs this weekend, too, and see what they look like. I need to check my split times on that last run and see where I lost the most time, bottom or top end. Keep me posted Paul.
 
I know my car was running lean when I got on it it run good and shot up to 12lbs of boost then it fell on it's face. I usualy see 15+lbs of boost by 3,500 rpms and when I looked at my LM-1 it was going wackey way lean. but when the car cools down it seems to be fine or atleast get 100 times better:confused:
 
I cleaned the MAF today and took it for a ride and now it runs good but I am still seeing high 12's AFR in 3rd gear pull @ 4000 rpms WOT and 15+lbs boost I use to see mid 11's. So there is still something wrong with the car and I don't know what I am going to pull the MAF off of my moms car and try that tomarrow.
 
You need to beg, steal, or borrow a fuel pressure gage that you can watch during a run. Reving the engine sitting still in no way tells you if you are getting enough fuel in a high load, high boost situation. You could be experiencing a degredation of the current capacity of the electrical wireing to the fuel pump. You could have a bad temperature sensor telling the ECU that it is much hotter than it is. Just a couple ideas anyway. Good luck.
Danny
 
EEC-IV adaptive learning?

Food for thought.
Are you running your EEC-IV in 'open' or 'closed' loop? Maybe running in 'open loop' will keep A/F more consistent?

Are you datalogging the ACT and ECT?
As you know timing is taken out as temps increase,
Perhaps in closed loop with timing taken out 'adaptive learning' is changing the A/F ?
(then at the next high RPM run, 'adaptive learning' uses NEW learned A/F from reduced timing, and then runs lean?)

My SCT advantage software shows OEM timing decreases -8* at ACT = 180*F
and decreases -4* at ECT = 216*F.
These spark numbers are proportioned from some value say 0*F ?
 
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Open loop, closed loop? I have no idea. I'm at WOT going down the dragstrip except for launch and gear shifts. Going home on the interstate at partial thottle, usually with the throttle just barely cracked open, it barely lights up 1 yellow bar as opposed to 2 or 3 going to the track. I can't datalog, besides it's not legal to datalog in NHRA ET bracket racing according to the rule book. The ACT sensor is a new one. The problem seems to be related more to engine temps rather than air temps. I could try to increase fuel pressure a little bit but don't want it too high.
My fuel pressure gauge is mounted where the shrader valve goes so I can't see it during a run.
 
FWIW - the pcm strategy is that WOT is considered 'open loop' and the O2 sensors aren't even in the equation - everything is off the MAF transfer function and the WOT fuel table - adaptive learning doesn't apply here.

Sure sounds like a heatsoak problem - at the strip, does it pick back up after a good cooldown?



bird963
 
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Not to try to sell anything, but Mike, I'd recommend you invest in some kind of data logger. Something like the LC-1 and Aux box from Innovative Motosports. You can hard mount it in the car and data log your fuel pressure, as well as RPM, IAT, and more if you add more channels.
http://www.innovatemotorsports.com/products/MTS.php

This stuff from the makers of the LM-1 is reasonably inexpensive when you consider the huge amount of knowledge you gain about what your car is doing. (about $450 to get started)

At WOT you should be in open loop. Bascially the engine computer is just telling the system to throw fuel at it and monitoring knock. It ignores the oxygen sensors. I suppose you could see an issue if the EEC didn't go into open loop on WOT. Not sure if it would show up as the issue that you're seeing though.
 
bird963 & Mike8675309,
Perhaps I misread Mike Wesley's article, below?
(ECT and ACT retards spark, to what degree is my question)

This is from www.fordfuelinjection.com

EEC-IV ADAPTIVE CONTROL
YOUR BEST FRIEND OR YOUR WORST NIGHTMARE!
By: Mike Wesley

yada, yada, yada....................
"What about Open Loop?" you might ask. Well, it works there too. This fact alone is know by very few people. Most people think Adaptive only works when in Closed Loop. This is wrong! Adaptive is only UPDATED during Closed Loop. It would be silly to ignore changes in the air and fuel system in Open Loop and only correct them in Closed Loop. If you have a serious fuel problem, your car might not even start if the EEC didn't have some way of correcting things all the time. The way the EEC uses Adaptive in Open Loop is similar to Closed Loop except it doesn't update the table. This means it's not looking at the oxygen sensor for feedback. It is merely relying on the information stored in the table to make corrections. Since the Adaptive table only contains Speed / Load points normally seen during Closed Loop, where does the correction factor come from if I'm at WOT? Good question. The answer is; it uses the last value it was using while in Closed Loop. Since the Keep Alive Memory has power to it even when the ignition key is turned off, the Adaptive table retains it's information. The only way to clear the Adaptive table is by disconnecting the vehicle's battery. Do that and you're back to working with a clean slate and the whole process starts over again. Now there are limits to how much the Adaptive Control system can change the calibrations...
 
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Since I haven't been able to get past the 2nd round at all this year I'm not sure how it would do after a long late afternoon cooldown. I do know that with a long cooldown before the 1st time trial it ran a 13.18 and then progressively got slower as the day wore on. I hose down the IC between rounds and it is cold before each run. If something is heat soaking, what is it that is actually heat soaking and why would it just start doing it? I'm leaning towards a failing sensor. I'll test both the MAF sensor and the ACT sensor this weekend and replace the O2 sensors.
 
sail7seas said:
bird963 & Mike8675309,
Perhaps I misread Mike Wesley's article, below?
(ECT and ACT retards spark, to what degree is my question)

This is from www.fordfuelinjection.com

EEC-IV ADAPTIVE CONTROL
YOUR BEST FRIEND OR YOUR WORST NIGHTMARE!
By: Mike Wesley

yada, yada, yada....................
"What about Open Loop?" you might ask. Well, it works there too. This fact alone is know by very few people. Most people think Adaptive only works when in Closed Loop. This is wrong! Adaptive is only UPDATED during Closed Loop. It would be silly to ignore changes in the air and fuel system in Open Loop and only correct them in Closed Loop. If you have a serious fuel problem, your car might not even start if the EEC didn't have some way of correcting things all the time. The way the EEC uses Adaptive in Open Loop is similar to Closed Loop except it doesn't update the table. This means it's not looking at the oxygen sensor for feedback. It is merely relying on the information stored in the table to make corrections. Since the Adaptive table only contains Speed / Load points normally seen during Closed Loop, where does the correction factor come from if I'm at WOT? Good question. The answer is; it uses the last value it was using while in Closed Loop. Since the Keep Alive Memory has power to it even when the ignition key is turned off, the Adaptive table retains it's information. The only way to clear the Adaptive table is by disconnecting the vehicle's battery. Do that and you're back to working with a clean slate and the whole process starts over again. Now there are limits to how much the Adaptive Control system can change the calibrations...


You just proved the point - it's NOT adapting to anything at WOT, just using the last correction factor and applying it into the rest of the equation based on MAF counts, the fuel table and spark table and it has been reportedly good for a little better 1/4 times when the battery has been disconnected prior to the runs.

Definitely want to look at ECT - feeds temp signal to the PCM and it will screw things up if it's failing when the engine temp get's higher. Granted, the PCM 'supposedly' only uses this signal for warmup info, but there have been many cases where failure of this sensor does cause drivability problems.

Bird963
 
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I just tested my ACT sensors. At room temp the original read 27k ohms and the installed one read 24K. After 10 secs of low heat from my paint stripper gun they both dropped to about .4K ohms. A spare 3rd one showed 25K at room temp. So I think my ACT sensor is working Ok. Visually it looks fine and wasn't crudded up any. I'll pull the MAF sensors from both cars and match them up later tonight.
 
The ECT sensor reads 26K ohms at room temp and about 2k ohms after 15 secs on the heat gun. My spare reads about the same. My MAF rensor reads 2.65 meg and 3.9k respectively. My spare again reads about the same. So those 2 sensors appear to be Ok. I pulled #2 plug and it was pretty white so it is definitely run a bit lean which means that my O2 sensors are Ok because they are indicating a true lean condition. I did find the clamp holding the inlet tube to the throttle body was loose and it was a tiny bit dirty inside the opening so it may have been sucking air in there causing a lean condition.
 
It could be a variety of things but most likely it's in the tune. The chip can manipulate learning any way it wants to, usually it's turned off for WOT but it might still be active (depends on the tuner). Best guess is that adaptive is compensating for a rich condition under cruise and applying that data to WOT.

There is a common misunderstanding that if the car is lean at WOT it should be lean at cruise and vice versa. This is not true. The EEC will always run at 14.7 AFR in closed loop. This is not negotiable! The stock O2 sensors cannot create anything other than 14.7 AFR so closed loop MUST = 14.7 (actuallly 14.64). If the EEC cannot achieve 14.64 AFR then it will eventually set a code for a non-switching 02 sensor. This will not happen right away and can take up to a few days depending on use and conditions.

The EEC will apply up to 20% fuel correction through the 02 sensors to achieve that mixture. After a certain amount of time that (up to) 20% correction will then be applied to the long term trims making the short term trims (02 sensors) -0- trim. This is how closed loop works.

The stock EEC will then use those learned long term trims to affect WOT by up to ( you guessed it) 20%. (!) So if your car was tuned to 12.00 AFR the EEC can possibly alter that to as much as 14.40 AFR at WOT.

Typically this happens when tuning is done only under WOT conditions. The tuner artificially corrects the WOT fuel table so that the car runs fine on the dyno, but if you don't turn off the adaptive strategy, then any change in the fuel trims will directly affect WOT AFR. This is why it is necessary to turn off adaptive strategy as it applies to WOT.

Tuning an EEC IV car (all SC's) under part throttle conditions is not easy. The absence of good datalogging information makes it difficult to determine what the fuel trims are and under what conditions. Particularly if you have an 89-90 SC.

FWIW, the EEC does not use "the last value learned" under closed loop. It can apply any number of different values from different conditions to WOT or it could use just one specific value learned at one specific load and rpm. Because CAI kits often skew the MAF function based on airflow, the assumptions that were valid for a stock car are no longer useable and therefore application of learned data should not be allowed in open loop.
 
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