1991, 4.0L, 286,000m, well maintained.

Problem with a stumble on accelerator tip in after cold start up, okay after warm up.

Perhaps associated is newer problem with starting. Engine would in past, turn over 2-3 times and start right up with smooth idle-cold or hot. Now, tries to start after 2-3 turns then dies. On immediate retry, engine will crank and crank without any attempt at firing. Will start if I floor the pedal and crank it over half a dozen times, but has rather lumpy idle for first minute or so and...I have a noticeable rich fuel smell. Still wants to stumble and die after putting in gear but a little fine feathering of the gas pedal will coax it past this point. Once up to speed, no problems with power or shifting. Once warmed up the stumble goes away and there are no problems with warm restarts.

I have cleaned the MAF, throttle body and IAC with no changes. The IAC is 'relatively' new and was deposit-free on inspection.

I am leaning toward a fuel pressure regulator problem but I can't see why the problem goes away after warm up. Any thoughts appreciated.

(on edit): I pulled codes this morning and received a message for 22 - "MAP or BP sensor-signal voltage out of specification(engine off) or not at normal vacuum levels (engine running)." A '91 uses a BP sensor for altitude adjustments, its a simple barometric switch with no vacuum lines. Don't know if this is relevant or not.

 

Cold stumble during tip-in is most often caused by lean conditions. When the engine is cold, it needs a richer mixture to run smoothly. When the engine warms up, it can run on leaner mixtures that are closer to stoich. So I would look for things that can cause lean running, such as vacuum leaks, low fuel pressure, or sticky fuel injectors.

I have the opposite problem on mine; after sitting for about a week, the engine starts almost immediately, within the first crank. But it won't do this after shutting off for a few hours or even a day. This leads me to think that maybe the fuel injectors are leaking, and pooling small amounts of fuel while the engine is off. I'm trying fuel injector cleaners now to see if that will change things.

 

well, unfortunatly '91 aero is OBD-I so obd data is not available, so my advice is to measure ECT voltage when engine is cold and has some miss and when it is hot. please post measurments here. Sometimes ECT works like having shunting resistor inside ore bad wiring insulation, corrozed connector... whell, voltage on terminals lower and ECM concider cold engine warma and according strategy actuate lamda controll (oxy sens. controll) and mixture is normal on cold engine. try to disable ECT. You will get CEL and ECT failure related code, but ECM will consider that engine te,perature is about 60 dgr C, so you will get no miss, but richer mixture (worth MPG) and faster idle.

 

holding WOT on OBD Ford EEC systems shuts off the injectors so no fuel. with the rich raw fuel smell, it's too rich on start until you crank it over multiple times to flush out the raw fuel. cold fuel wet plugs start hard especially in your high altitude cold thin dry air. lot less oxygen at 7k feet. bank injection of the OBDI 4Ls only make the problem worse.

this ethanol adulterated goop they call gasoline only makes the matter worse. you may have got a dump of gas meant for the warm south or the tank farm forgot to mix in the winter adds for a load. low vaporization additives makes a gasoline worthless in cold weather.

http://www.turborick.com/gsxr1127/gasoline.html
5.2 Why are there seasonal changes?

Only gaseous hydrocarbons burn, consequently if the air is cold, then the fuel has to be very volatile. But when summer comes, a volatile fuel can boil and cause vapour lock, as well as producing high levels of evaporative emissions. The solution was to adjust the volatility of the fuel according to altitude and ambient temperature. This volatility change has been automatically performed for decades by the oil companies without informing the public of the changes. It is one reason why storage of gasoline through seasons is not a good idea. Gasoline volatility is being reduced as modern engines, with their fuel injection and management systems, can automatically compensate for some of the changes in ambient conditions - such as altitude and air temperature, resulting in acceptable driveability using less volatile fuel.

pablo can tell you how hard it is to start a fuel flooded engine at -20d C and keep it running when too rich.

too much fuel. ECT or IAT sending incorrect info to PCM
MAF has some input at cold idle no throttle input start. no input by O2 sensor on cold start, cold drive away.
my money's on an ECT not knowing when it's below 40d F.

my power miss is back today. thought i felt it yesterday but far worse today. if fact worst ever. barely pulls hills on the freeway.

ran so nice and had so much power for several months. at least i know it's not the tranny going out. runs like 2 plug wires fell off, all looked ok.
when i'm over being upset at it, will take the scanner out and read the stupid computer. maybe a semi will hit it parked out on the street.

 

Engine seems to be too rich on cold start. Unusual problem, so you need to check out the cold start circuit. I would crank it once untill the apparent flooding condition occurs. Pull out a plug and see if its wet. If it is you need to check out all causes of too much fuel spray. To see if the problem is temp related can you park it inside? ACT & ECT have a huge effect on fuel spray (inj on time) in sub zero conditions. ECT/ ACT signal voltage should be close to 4-5v with temps below 30f, at 200f it should read 0.5 to 0.6 volts, both sensors will read about the same voltage relative to the measured temperature.
Could you also have a leaking injector(s) or malfuncion in the FPR causing overpressure in the fuel rails? However if this was the case you would also have engine problems when warmed up too. Air filter/ intake is not restricted in the airbox? (Leaves or mice, etc).

Make sure plugs are good! cold plugs dont fire as well as warmed plugs/engine.

 

So, that't two votes for the ECT, one for FPR and I don't know on third.

Stumble occurs whether the van 'cold' starts from inside a heated garage or outside at cold ambient temps.

If the ECT were shorting or otherwise misbehaving, why the changeover when warm? Could an initial bad reading from the ECT cause the ECM to select the wrong cold start mode then switch to a different program mode as the various warm input sensors come online? I would really like to believe it is the ECT because it would be simple, cheap fix. I think I have a spare on the shelf, I'll give it try over the weekend and see what happens.

 

look! EEC-iV is not so suffisticated. there is a table of strategy here Ford Fuel Injection Strategies

over 180 dgr no change in table.

but first check ECT with AVOmeter, it is easy! compare ECT voltage table and real temperature. you may find dfference ore xoncider ECT to be OK and look for other problem

 

Just ashot from the hip you could also try just try disconnecting the O2 Sensor and see if idle changes. If it doesn't could be a bad O2. Just a WAG.

 

Yeah, disconnecting some sensors may let locate problem. I had lazy O2 sensor syndrome. Did not work cold. Really lambda controll is disabled below zero drg C. I cleaned O2 sensor with solvent. still works.

 

O2 is not in the loop in cold start/cold driveaway PCM modes.

PCM tests the O2 sensor for heated on ready and then recognizes O2 sensor and goes into closed loop warm drive/cruise mode using the O2 sensor readings after coolant ECT hits about 180d F.

cold start up PCM operation is only MAF, IAT and ECT. i almost forgot the cripple BPS used on the first 4L OBDI systems

me thinks the ECT has failed on the cold temp end of scale or it's that cripple BPS that has failed and thinks the rig is at sea level with thick air and tons of oxygen, dump in the fuel.

has your warm cruise fuel mileage changed?

 

well, I would agree, cold loop fuel correction may be done using hot loop O2 data. so try to reset RAM disconnecting battery terminals tonight. sometimes everething work fine after that

 

Been away for the weekend.

The weather here has been unusually warm. Haven't noticed any stumble over the weekend. Yesterday I tried a different (not new) ACT sensor and have only driven to work this afternoon. No big changes to report at this time. I'm going to clear codes and see if the BP error pops up again. Perhaps it does think I'm in Florida instead of Colorado.

 

try ECT (coolant sensor). ACT is used for air mass calculation (together with MAP) on old non-MAF ECMs. So if ACT data is wrong a bit it corrected with lamda controll unction

 

My 94 would go nuts once in a while.. Seems it would act up if the prev day it was warm and the following day got cold.or the other way around. Go to start it and it would get really rich etc. Didn't happen too often but it was weird. The 96's don't have those issues..


Dick