Engine Cold start problems due to sensor? But which one, no parts cannons!

beeduckworth · Aug 20, 2023

First post. I have 1990 gt and I completely rebuilt the motor. It runs poorly on startup like multiple misfires. Once it warms up it runs smooth as hell. I have tried a lot of parts testing and diagnosing. What I see happening is something in opem look causes this to run like crap. I've searched forums for days and can't find what exact sensors are being used in open loop. All I know is if I cold start is and It running terrible, all I do is unplug ect sensor and engine runs perfect. Plug ect back in and engine runs like crap...can't figure out what that means, bad maf? When I unplug ect does that send it to closed loop instantly? If so it runs amazing. I just don't know enough about all the sensors and computer based on cold or warm engine. Any ideas.?

vferrizz · Aug 20, 2023

I think most folks on this site will recommend warming up the engine to normal running temperatures, shut it down and then pull the KOEO codes. Either use a code reader or do it manually by installing a jumper between the top right terminal of the test connector and the other end to the single wire STI connector. I am starting to prefer pulling the codes manually since I have been using a code reader but it's been showing me really odd things lately so doing it manually eliminates the possibility of a bad code reader.

There are tons of great posts on this site that explain what all the sensors are for and how to check them. Need to pull the codes fist tho for starters.

beeduckworth · Aug 20, 2023

You asked to pull codes when warm, well car runs perfect when warm. How will that explain why it runs bad when cold? I have plenty of cut and paste posts to tell me how to test all sensors useing my dvm and they all are in spec. Again my question is what does the engine use as sensors in open loop to control idle? Is it just the maf? If so then this must be my problem.

vferrizz · Aug 20, 2023

Idle control uses the PIP and ECT sensors. PIP sensor provides engine RPM input to the ECU and controls idle speed using the IAB. ECT sensor tells the ECU if the engine is hot, warm or cold. PIP sensor provided the feedback. PIP sensor also provides feedback to the EGR control closed loop system.

beeduckworth · Aug 20, 2023

I see in Sept of 22 you have been drug through the mud on your own idle problems so if anyone is familiar with idle problems it would be you. But diagnosing is a funnel of findings leading to one or more issues that once repaired will solve the issues. I'm seeing that your responses don't seem to directly relate to my issue. Pip is the only thing that controls idle during open loop. Well I find that hard to believe since the computer would need to know how much air is coming into the engine(like maf). I understand that the iab plays a role , but how much? It looks like the guy I bought this from used all aftermarket China sensors, so this is where I think I'll start over with all either motorcraft or standard motor products brand sensors and go from there. I've been screwed by way to many knockoff sensors to be wasting time chasing my tail for a month. I'll start with maf, it can be the only issue that tells the computer how much air is flowing in to the adjust fuel properly. Change my mind...

AeroCoupe · Aug 20, 2023

ECU is in Open Loop when the engine is cold i.e. when you first start it due to the ECT sensor (the one in the crossover tubes between the lower intake / water pump and the heater core) not being at the minimum temperature to switch to closed loop. It does however use the MAF, ECT, and ACT (the on in the lower intake on the #5 runner) sensors in Open Loop. The reason all of this is important is that in open loop the ECU ignores the O2 sensors and delivers fuel based solely on the tables in the ECU. When the motor reaches the minimum temperature (ECT provides this input) the ECU switches to closed loop and uses the O2’s to trim the fuel.

You need to pull the codes as the ECU will tell you with is not making it happy.

beeduckworth · Aug 22, 2023

Code 34 and 85, I have smog stuff removed so 34 is evap and 85 is charcoal solanoid...

AeroCoupe · Aug 22, 2023

KOER or KOEO or both?

beeduckworth · Aug 22, 2023

Key on engine off after warning up

AeroCoupe · Aug 22, 2023

You should also pull codes with the key on and engine running.

beeduckworth · Aug 22, 2023

Same codes I just did that, I get a 10 then 34 85

Bullitt347 · Aug 22, 2023

My guess is that the fuel injectors are bleeding fuel once the engine is shut off and it sits. Then when you go to start it, the cylinders are rich and it is causing a missfire and once the excess fuel is burned off it starts to run normally.

beeduckworth · Aug 22, 2023

Can't be, I can pull ect and force it into closed loop and misfires go away instantly, within seconds of cold starting. Then plug back in and it'll run like crap again. Or if I leave ect plugged in and just unplug maf it stops misfiring and runs smooth again. I think it has to be the maf. I measured voltage when running on maf sensor and it reads .5 volts, looks withing spec based on a helpful sensor sit for foxbodys that tell you how to test all of them.

General karthief · Aug 22, 2023

Why is the charcoal canister and valve removed?
Not that it will have nothing to do with your issue.

beeduckworth · Aug 22, 2023

Bought it that way

General karthief · Aug 23, 2023

beeduckworth said:
Bought it that way

Check if the fuel tank vent hose is still in the dog house (engine compartment). If it is I would put the canister system back in, if not it's decision time.
The advantage to the system is the fuel system is close (basically) and no vapors escape stinking up the garage and a possible fire hazard.
It also fatten up the cruise mode by design.
It is an 'emissions ' thing but has no negative effects.

Kid wita 5oh · Aug 24, 2023

Just a thought, if you can unplug the MAF or the ECT and the issue goes away... Have you checked your VREF circuit? And check the ground from the engine to firewall....

AeroCoupe · Aug 24, 2023

So the Code 10 is the spacer code that the computer puts out just before it starts to dump the rest of the stored codes (jrichker quote) so looks like you are chasing a wiring problem. Does the car have an EGR or has it been removed hence the Code 34? I went back and reread the thread and did not see this mentioned.

Mustang5L5 · Aug 24, 2023

Check for vac leaks. Do a smoke test. This would cause poor running while the car is warming up.

beeduckworth · Aug 26, 2023

I checked vacuum leaks with a smoke machine, nothing. Yes it does still have an egr but maybe it's not functioning well. What confuses me the most is while running poorly, if I unplug the ect then the car runs perfect during cold start/ warm-up. I don't see egr as an issue or it would be an issue with ect unplugged....correct?

Engine Cold start problems due to sensor? But which one, no parts cannons!

New Member

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lube between the nut and the face. I know my lubes

New Member

lube between the nut and the face. I know my lubes

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lube between the nut and the face. I know my lubes

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I have been doing it wrong this whole time

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wonder how much it would cost to ship you a pair

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wonder how much it would cost to ship you a pair

I'm definitely not in the original hole

lube between the nut and the face. I know my lubes

That is…until I whipped out my Bissell

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