OK!

SO... new to the forum. Not a mechanic but I know a little bit. I signed up specifically to post this issue as so far it has been unsolvable. Hopefully someone here can help! I have researched the crap out of this on the net and in the haynes manual, and asking around.

Vehicle: 1995 F-150 4.9L, L6, 240000km (150000 miles), standard transmission

Symptoms: A surging / Rolling Idle, and misfires at about quarter throttle. At WOT, or under hard accel, no issues misfiring. Also crappy mileage but I know why (stuck in open loop, easy fix)

OBD-I Codes:
116: ECT out of self test range
172: Heated Exhaust Gas Oxygen (HEGO) sensor - voltage signal indicates "lean"

I have checked and ensured proper voltage or resistance following Haynes manual specs on:
Idle Air Control (slightly higher than spec voltage, should be 10.5, reads 12)
Throttle Position Sensor (bang on spec)
Engine Coolant Temperature Sensor (resistance cold and warm is accurate)
MAF: There is no MAF on these models, that I was aware of.
EXCEPT the Manifold Absolute Pressure Sensor voltage is reading funny. More below.


I have replaced throttle body, IAC and EGR gaskets, thoroughly cleaned the IAC and throttle body. Checked for vacuum leaks with carb cleaner and small propane torch. I might also add when I introduce a vacuum leak, it simply idles on average higher, but still surges and rolls. PCV valve rattles and I can blow air through it. I've tried the EGR flow limiter as suggested elsewhere on the net, but all it did was throw a code for it and was no help.

Also new plugs, distributor cap, rotor, fuel filter, oil & filter, did a complete coolant flush and refill.

Worth noting if I disconnect either the MAP or TPS, the idle levels out and is stable, though is a little rough.

Alright! So moving on to known potential issues.

Code 116: ECT out of self test range. I think the thermostat is stuck open. I'm going to replace it asap.The lower rad hose increases in temperature at a linear rate with the engine, indicating it's open. This would indicate why the PCM is also stuck in open loop mode - it can't get up to operating temperature. This doesn't explain the idle or misfiring entirely though.

MAP Sensor Voltage: On testing the MAP reference voltage on the connector (off the MAP), I get 5v as specified. Then connecting it onto a brand new MAP, ignition on and engine off, no vaccum, it reads 2.67 volts. It should be 5. Under vacuum conditions it reads the same 2.67v. HOWEVER, it's not throwing a code 129 or any other for that matter. This seems really strange and I can't rationalize why. Thoughts are appreciated.

Otherwise, I'm completely stumped. The ECT code should be resolved with a thermostat, nor should a thermostat cause these problems (or running in open loop). The MAP sensor isn't throwing a code (it's brand new) BUT it is reading wrong voltage according to Haynes. The O2 Sensor code doesn't matter at this point because the engine is stuck in open loop, so it's not using that data to idle at or run while in open loop mode. Everything else seems to be in working order. Possibly still a vacuum leak? PCM Fault? The snap-on scan tool i'm using doesn't seem to think so...

I realize this might be a lot to take in, but if anyone can help solve this I'd be super stoked!

Thanks!

 

MAP sensor output is variable frequency square wave. It's not a simple DC voltage.

Code 116 can be triggered if you do not warmup the engine before running the KOEO or KOER tests.

My take on Code 172 is you are running lean. A rolling idle can be caused by a vacuum leak, bad TPS or sticking IAC. You already replaced the IAC that leaves the TPS and vacuum. These 4.9L engines are notorious for leaks at the intake manifold. I realize you tried the spray method and propane but I find those methods sketchy at best.

 

Hey RLA!

The 116 code was after being warmed up for a few minutes and driven for about 6-7 miles. Also there is very little movement on the dash gauge which tells me it's not getting to full running temperature. Am I also right that there is a separate sending ECT to the dash? I am testing the one at the front/top of the block. All being said, I would consider this a separate issue from the missing & loping idle.

I dug up some other threads on the MAP voltage... indeed does run variable frerquency! I was pretty stumped on it, so thanks for that tip! I was pulling a code 129 (bad MAP) before hand with the old one, so it needed to be replaced anyway.

The TPS has also been recently replaced. I also double checked the voltage and return signals, they are fine, plus back probing - it all checks out so I think we can safely rule that issue out.

Which brings me back to vacuum leak (again). Would you recommend taking it to a shop to find a potential leak? Or is there a method I'm missing? I've replaced the throttle body, IAC, and EGR gaskets - do you think maybe the intake manifold gasket? Gaskets are cheap so I don't mind throwing those at the truck.

Double check my logic is sound: Loping idle and 1/4 throttle misses - it wouldn't miss at WOT throttle because there is so much air entering the manifold it's reducing the vacuum leak to negligible. When I disconnect the MAP or TPS at idle, it levels but is rough because it's not using the MAP data to fuel the engine - so it's not flying all over the place trying to compensate for a constantly varying manifold pressure. The lean code also suggests too much air is entering - but it's stuck in open loop so won't use the O2 data to compensate yet so will always have this issue until it the loop closes or the leak is fixed. Good logic?

 

So bit of an update, I think it is a vacuum leak.

Managed ad-hoc build a smoke machine and ran it into the intake manifold. Serious leak out of the EGR valve... I'm guess it's stuck open /partially open, and needs replacement. Quite a bit of smoke coming out of it. This might explain why the cab smells like exhaust.... it appears like there is just a hole where the exhaust is venting and the vacuum leak is...

I took it off and put my finger over the hole where the EGR vents into the manifold to seal it up, and saw some wisps of smokes coming out of the side of the throttle body where butterfly valve control is... not much and it held it's pressure ok.

Also in the mean time, would sealing up the EGR have any negative repercussions?

 

Some folks report issues with the EGR blocked off, others do not. Personally I like to keep it working.

There should be no smoke from the throttle body shaft. It's a vacuum leak.

 

I also bench tested the EVP Sensor just now.

3600 ohms resistance, 400 plunger depressed (VREF to EVP Sig)

75 ohms EVP Sig to SIG RTN. 3600 depressed.

Haynes list "Slightly below 5500 ohms, and no less than 100 ohms"

EVP sensor shot too or would this be a reasonable range? According to Haynes it's not.

 

I rely on the voltage output: http://oldfuelinjection.packrad.net/page35.html

 

Gotcha, I'll check that out in the morning.

Reading up some material I understand that smoke coming out of the EGR is normal if it's a positive back pressure type. Anyone confirm this is the case, or rather do the 4.9's use this type of EGR?

 

I am not aware of that term nor have I considered smoke coming from the EGR valve normal during a smoke test. Others may have input.

 

So I back probed the EVP - Keys on engine off. I pulled the sensor from the EGR to make it easier, i figure it should run the same.

I tested 0.001V (basically 0), plunger out, and 4.85v, plunger depressed. I have the light grey sensor.

The chart for voltage says at 0% open (plunger out) voltage should 0.5v, and fully depressed (100%) 3.9v.

In your opinion, bad EVP sensor? It would appear it's out of test range.

 

The EGR valve preloads the EVP sensor. You need to install the EVP on top of th EGR valve.

 

I've safely ruled out the EGR and EVP as a vacuum leak. I've essentially sealed off the EGR with two gaskets and a tin plate cut to size sandwiched between, cutting it off entirely. I disconnected the EVP sensor as well.

The engine still runs the same. I also ensured it's not the IAC again. I disconnected it and set the idle screw high enough it won't stall out when disconnected - it still surges with no IAC.

I took off the throttle body and pressure tested it by sealing up all the outlets except one to pump air in. Admittedly it doesn't hold 100% pressure, it's not bad. I would have a hard time seeing a vacuum leak this small causing this much problems.

One of things that garnered an interesting result was is pulled the vacuum line off the MAP while running, and plugged it with a finger. I applied with a vacuum pump standard idle vacuum to the MAP. The engine ran 100% fine.

My theory would be that the MAP isn't getting a constantly changing vacuum, so the PCM doesn't change conditions to compensate, surging the idle - all suggesting a vacuum leak again.

I sealed off the manifold after the throttle body and tested pressure again. It bleeds it off within a few seconds somewhere still.

As of now, still hunting for a major vacuum leak...

 

I have suggested before to cap of all vacuum ports except the MAP sensor. Start up the truck, if you still have a surge then the leak is on the engine. If no surging it's external, start adding one vacuum circuit at a time until it starts surging again.

 

Awesome, great advice. I've determined that it's not an external port, as it surges with the vacuum tree closed off (pretty sure I got everything - main vacuum tree, pcv, and throttle body vacuum lines all closed off)

So it's the engine that has a vacuum leak. This is a little outside my knowledge range. Other than the intake manifold gasket (upper and lower, correct?) what else is there and how could I diagnose this? Smoke test again?

Thanks!

 

Make sure the power brake booster is not connected. Common source of a vacuum leak.

Someone else recently stated they used the smoke test and found multiple leaks at the manifold to head connection.