Funky OBD-II readings...bad TPS?

Contour Enthusiasts Group Archives

This is a spin-off of my orignal thread.

I hooked up my laptop today and got some weird readings. With the key in the "on" position, and not running, the TPS is at 16.5% and the MAF was reading 0.117#/min. Shouldn't both of these values be ZERO if i'm not touching the gas pedal AND the engine is off?!?

This would explain why my car won't idle without dying. The computer thinks that the throttle is open so it is dumping gas into the engine and closing off the IAC. This is drowning my Zetec in fuel and not giving it any air. I've gotta swap out that TPS!!!

BTW, what kind of readings should I have for my MAF and TPS at a given RPM?

Here are some of my other readings...

Engine off:
MAF=0.177 #/min
TPS=16.5%
ECT=73 degrees F
IAT=45 degrees F (It was about 40 outside)

Started and holding gas to maintain 2k RPM:
MAF=1.113 #/min
TPS=20.8% (only 4% change?)
ECT=180
IAT=54
LOAD=25.9%
Short Term Fuel Trim=varied from -1 to 0
Long Term Fuel Trim=3.9
Spark Advance = 35.5 degrees
#1 O2 Sensor = varied from 0 to 0.745
#2 O2 Sensor = 0.725

Revving to 3500RPM:
MAF=1.702 #/min
TP=21.6% (less than 1% !?)
Spark Adv = 36.5 degrees
LOAD=29.4%

Idling badly:
MAF=0.366 #/min
TPS= 16.5%
ECT= 185
IAT= 66
LOAD= 41.2% (MUCH higher then at 3500RPM)
Short Term Fuel Trim= varied from 0 to 2.3
RPM = 800
Spark Advance = 28 degrees
#1 O2 Sensor = varied from 0 to 7.35
#2 O2 Sensor = 0.818

FYI, all readings were done with my car in park.

BP

Not sure about minimum MAF % but don't think it can reach zero.

16% TPS is bogus for sure.

You have been resetting the PCM, right?

Never heard how it runs other than idle. Timing belt OK? Checked the vacuum?

Yep, reset the PCM everytime. Vacuum is good, and the timing belt only has 17k on it.

I'm gonna yank the TPS tomorrow and substitute a 5k potentiometer and see what happens.

Waddya think?

BP

Sounds like a plan - since you put everything but the kitchen sink in there already!

The following is from the Ford TSP (Technical Service Publication):

MAF voltage @ KOEO (Key On Engine Off) should read 0
MAF voltage @ hot idle should read .6-.9
MAF voltage @ 30 MPH should read .8-1.5
MAF voltage @ 55 MPH should read 1.2-2.5

TPS voltage @ KOEO (Key On Engine Off) should read .53-1.27
TPS voltage @ hot idle should read .53-1.27
TPS voltage @ 30 MPH should read 1-1.3
TPS voltage @ 55 MPH should read 1.1-1.4

Screw it, I'm done.

I just checked the TPS and MAF voltages, both are within spec.

I'm gonna suck it up and take it in tomorrow.

BP

the tps isn't designed to go to zero, but the MAF will be close if the engine isn't running.. If you have an idle proble, it is most likely caused by the IAC. Also, the IAC opens in proportion to the throttle opening to act as a dashpot (like on older mechanical carburetor engines) to prevent gaskets from being sucked in and failing in the event that you snap the throttle closed at high RPM.

My readings are on my laptop at work, but i know that the TPS was around 0.62. The MAF was also in range.

Besides putting the old TPS back on, or buying a new one, is there anyway to test it?

I'm guessing that I would need and oscilloscope in order to determine if it is running at the right voltage and frequency. Ugh...I am sooo sick of working on my car...

...maybe tomorrow...

It's satruday night. Time to go drinkin'.

We have a 99 Zetec ATX. The min throttle position setting seems to always read 17%. There are no idle problems (yet!)on our car. I don't think you have a bad TPS.

Yeah, with the TPS at 16% it read somewhere around a volt (well within limits).

I'd like to try swapping out the IAC but I really doubt that it is bad. It worked flawlessly when i had it out.

(I hope this made sense..Jeagermeister(sp?) is screwing with me a little.)

BP

Am I missing something? Did you check if the fuel pressure is high?

It varies between 30 and 40psi. It is usually about 35 when idling but never goes under 30psi.

You have not taken it anywhere yet???

Did you replace the tensioner with the timing belt? You really should verify static timing.

Unless the IAC is not getting a signal from the computer, don't know what else to do. But the rich thing does not really go with that. Shouldn't run rich just cause the IAC does not open enough. The timing could, but manifold vacuum should be low - maybe only 15 in.

ButtonPuncher: I have the reference values chart from the Ford DVD. I can email it to you. It lists several items and what their values should be at KOEO (Key On Engine Off), Hot Idle, 30 MPH and 55 MPH.

Listen to Rogerm60, too. My problem turned out to be mostly incorrect static timing. Even though I was already checking it when he posted the idea, he was the only one that ever recommended looking into it.

EDIT: Nevermind, finally got the FTP client to upload it. Right click - Save me... It's for a '98 w/ MTX, but it should be able to give you the big picture.

My Zetec shows a minimum TPS of 17.6% and a minimum MAF of 0.177 #/minute when the engine is not running. A Duratec that I recently examined showed a minimum TPS of about 18%. I think this must be normal.

Perhaps the scaling of the TPS range supposes that 0% would be completely closed, so that no air at all can go through �?? a position which, for obvious reasons, should not occur.

WOW! Thanks for the input everyone. Here I go...

Rogerm60,
No, I still haven't takin it in yet. I just got promoted last week and have been working my A$$ off. I just haven't had time. Anyway... I had my timing belt replaced just after I bought the car (at 87k, now I have 100k). I doubt that they replaced the tensioner. How can I verify if it is working properly? What do you mean by static timing?

The one piece of test equipment that I don't have is a vacuum guage. If I buy one, what should my readings be at idle?

D Davis,
Thank you, thank you, thank you, thank you, thank you for the OBD readings! This is what I've been dying for. Finally something to compare to. Did you actually fork over the cash for the diagnostic DVD?!? You are HARDCORE!! ...either that or you work at a dealership... Thanks again!

Bob,
Those readings are EXACTLY the same as mine. I feel much better now.

Thanks again everyone!

Now please tell me about static timing and vacuum levels. I wanna fix my baby!!

You're welcome

I bought the DVD from Bill Jenkins last year. I think it was about $25 through him.

Static timing is the physical alignment of the cams and crank. I just redid mine this weekend. The car runs better, but I have a CEL (P1383 - VCT OverRetarded) and what feels like a loss in power below 3k rpms. You can read the progression of my issue here.

Originally posted by D Davis:
I think it was about $25 through him.

WHAT!?!

I thought that it went for about $1200. Thas on heck of a discount. Damn, I want one.

Off to read your thread...

BTW, does anyone know what my vacuum level should be at idle? What about at 2k RPM? I gotta go buy a guage.

Later,
BP

D Davis,
Just got finished reading your thread. The static timing problem sounds VERY interesting. But I doubt that that's it because it was an intermittent problem that turned into a constant one. I should check it out tho. I'll have to bust out the service manual to see where the timing marks should be. Thanks.

Now about the vacuum levels. Anyone? Pretty please with sugar on top?

You've got to be kidding me! I have to take the freakin valve cover off to see it the cams are aligned!?! Why didn't Ford put some kind of marking on the cam gears?

Will a local dealership have that alignment tool? How much is it?

Questions, questions, questions...

BTW, just got a big-a$$ vacuum guage from Sears.

BP

Hahaha - you're reading as fast as I can type

Yeah, the cam cover has to come off because there are no marks on the outer edge of the gears.

You can make the alignment tool yourself. You need a strong piece of metal 9" x 1 1/2" x 0.20" - the length and width are approximate, but the height (0.20") is critical. I found an L-bracket at Lowes and ground it down a little. Total cost - $6. If you bought it from Ford, probably $20.

And I don't know about the vacuum level. I bought the DVD, but can't find it

I have to keep calling Po-Jay up for info

Vacuum readings vary from engine to engine. Normal would be 17 - 22 in. at idle. It will go up slightly as RPM's do and spike high when the throttle closes. Vacuum should be very steady at idle. If cam timing or overlap is wrong, vacuum will be lower but steady (10 - 15 in.). Late ignition timing will also cause vacuum to be lower. In our cars ignition timing is not adjustable but it is tied to cam timing via the cam position sensor. Wandering vacuum readings usually are caused by vacuum leaks. Bouncing readings are caused by mechanical problems. Plugged cats cause vacuum to fall as RPM's rise.

Read several vehicles to familiarize yourself, including well running ones and other problem ones if you can.

D Davis,
Was your diagnostic DVD a special deal, or is that how much Bill normally sells it for? I'm kind of wary about checking the static timing. I really doubt that is the problem. My problem was intermittent and turned constant. I would think that the belt would only jump forward on the gears, not forward then back. BUT, I still really want to check out those OBD readings.

rogerm60,
Thanks for the vacuum info. My idle is so incredibly rough that I doubt I will get any meaningful readings. I guess that its still worth a shot tho.

Argh...maybe I will feel more optimistic on Sunday.

Later,
BP

OK here are my discoveries for today...

I ran the Enchanced Tests with my OBD-2.com scanner and got the following errors:

P1703 Brake Swtich Out Of Self Test Range
P1780 Transmission Control Switch (O/D Cancel) Circuit Out Of Self Test Range
P1550 Power Steering Pressure Sensor Out Of Self Test Range

The first two, I could see as bogus, but the last one concerns me. If the Power Steering Sensor was bad, would it cause my horrible idle? I would think that it would only cause my car to die if I tried turning the steering wheel when stopped.

For the other readings during Hot Idle, the only ones that were out of range were the IAC and Spark Advance values. The IAC was 37.5% (when it should only be 33%) and the Spark Advance was 27 degrees (when it should only be 15 to 25 degrees).

BTW I still haven't had a chance to hook up my vacuum guage.

BP

Those 3 errors will show up if you don't depress the brake pedal, then turn the steering wheel and finally hit the overdrive cancel button during the test when asked to do so.

My spark advance was resting at about 28-29 when I ran the tests. Just FYI

My 'tour is driving me insane!!! I started my car at work, wardmed it up, and it was idling like crap. I drove to my parents a mile away, IT WAS IDLING PERFECTLY! I was amazed. I ran back over to work to get my laptop, came back, and it was idling like crap again. Stupid intermittent #$^&#$#.

I am starting to wonder about my brake booster line. That is the only vacuum line that I haven't checked. I was trying to follow it around but it is diffucult to see because I have my stock intake back on. It looks like it connects to the bottom of the intake manifold. Or does it snake around somewhere else?

I believe it's tee'd to the PCV valve.

Mine runs from the brake booster to an area under the intake manifold. You're right about the stock intake getting in the way. I probably wouldn't have been able to follow mine if I still had that intake on.

The length of the line is about 24-36" (rough guesstimate). The Haynes manual mentions pushing it in towards the intake manifold and slowly pulling it back out to release the clips. I had no luck releasing it, so I left it alone

This job may be easier performed from under the car.

Anyway, the book notes you should only be able to blow through it one direction. I can't remember which way, though. Let me know if you get it off. I was afraid I'd break mine.

What about your fuel pressure regulator? Being a '96, you should have a return fuel system. Is your vacuum line on the FPR seated well?

OK I hooked up my shiny new vacuum guage tonight. I "T"'d it to my fuel pressure regulator. (It was the only accessable vacuum line with my stock intake back on.)

Here are my findings:

Idle = 7-8in (in drive with brakes on, around 600rpm)

Crusing = 15in (flat road, 45MPH, 2500-3000rpm, O/D off)

Completely letting go of throttle at WOT = 22in

Now I am really wondering if my brake booster line has a crack in it.

Waddya think?

BP

Low idle vacuum:

Major vacuum leak
Late ignition timing
Wrong static (cam) timing
Low compression
Mixture way off

Of course low compression is constant, ignition timing is non adjustable (unless it's always late because of a bad knock sensor), and static timing does not normally bounce around unless there is play somewhere like the cam sprocket bolts being loose.
Other things like collapsed lifters and sticking valves cause lots of vacuum bounce because it's only one cylinder doing it.
A large vacuum leak does not go well with smelling gas, nor does a leaky induction system causing a lean condition.

Keep thinking, and testing.

Thanks. It may actually get above 10 degrees this weekend, so I may have a chance to finally fix this.

I'm banking on the brake booster line being cracked.

I've been thinking more and more about it lately. At low rpm's it idles like complete and total crap. At high rpm's it runs GREAT. I mean even better than it usually runs. This would imply that I have unmetered air getting into the system, which would also be caused by a massive vacuum leak. It would cause my engine to run lean at higher rpm. This would actually help the high end performance but obviously hurt low end performance.

Last spring, I made a home-brewed intake out of that aluminum dryer hose stuff. It worked really well until the days leading up to it tearing open. I noticed more power in the upper rpm band and didn't know why. But one day when I came to a stop light my 'tour just died. I started it up but had to give it gas. I don't know why I didn't think of this before.

Enough of storytime, time to go get some

Its been a while..so here's an update (if anyone cares

).

My problem is definitly cold related. Ever since it has gotten out of the 30's, my car has been running just fine. There have been a couple of cold snaps and I will notice a little of the bad idle.

Tonight when I came home it was REALLY bad. I just wish that it would stay broken so that I could take it in and get it fixed.

Hopefully I will accidentally fix it through ripping my engine apart this summer and replacing all of the sensors that I haven't gotten to.

Later,
BP