My '93 Bronco is having a starting issue. It's got a 302 V8 and everything is stock. It's got 197,500 miles on it. I'm trying to get this truck in top working order as it's going to my daughter.

It's taking longer and longer to get the engine to start. Like 5-6 turns of the ignition to get the engine to start. Once it starts, it runs like a champ.
Fuel pressure at the rails is 35-40 psi.

I changed out the fuel pump, fuel filter, spark plugs, rotor and cap. Still takes 5-6 times to start.

It's now at the shop and after a week they have ordered a MAP sensor to try, but they told me today that they're baffled.

Anyone run into this problem before?

Thanks,
Brian

 

Second time in as many days that I've posted this. ECT and/or ACT sensor issue. These two sensors do not necessarily reveal any "problems" (fault Codes generated by the on-board computer) when they screw up.

I've explained this before but perhaps I should do so again. (Note to self, add this to the FAQ website).

The ECT (Engine Coolant Temperature) and ACT (Air Charge Temperature) sensors are two of the three thermally reactive sensors in these trucks. (The third is the HEGO sensor). As these two sensors heat up their resistance value changes. The computer sends +5VDC OUT to every sensor in the truck (except the HEGO). As the resistance value of the sensors changes, the computer "interprets" the changes coming BACK to it and makes adjustments to the operational parameters of fuel/air ratio and spark timing.

In the case of the ACT sensor, it is reading the temperature of the air coming into the intake. This is interpreted by the computer and alterations are made based upon what effect the incoming air temp will have on the combustion cycle. This sensor is more greatly affected by the ambient air temperature the truck is operating in than actual engine temperature. If the truck has trouble starting or running only in one ambient temperature extreme, this sensor is the more likely culprit. (i.e. only has trouble in very cold weather)

In the case of the ECT sensor, it is reading coolant temperature. This gives the computer an "idea" as to how hot or cold the engine is. (Think of it as the thermal choke setup on an old carburated engine). As the temperature of the coolant rises, the resistance value of the ECT sensor changes. This, in effect, "tells" the computer that the engine is warming up and we all know that a warm engine needs less fuel to run properly while a cold engine requires more fuel (and less air).

Consider for just a moment that the ECT sensor operates at a range of temperatures from dead cold (for argument's sake lets say 40º F minimum anything colder won't matter the fuel mixture will only get so rich before you flood the engine), to full operating temperature which is typically 195º F but for extreme situations lets say 250º F. So, the computer is programmed to look for a "temperature" in this range from the ECT sensor. If the information coming back to the computer is OUTSIDE this range, you should see Code 51 or 61 with an ECM generating two-digit Codes or Codes 115, 117 or 118 with an ECM generating three-digit Codes.

But what happens if the engine is actually dead cold but the ECT sensor is "stuck" at a "temperature" of 180º F? Well, its simple, the computer "thinks" the engine is a LOT warmer than it really is and so its sending very little fuel through the injectors. So you are trying to start a COLD engine with too little fuel. Hence the hard start situation. The problem with troubleshooting this is that since the 180º temperature reading is "within operational range", the computer doesn't throw any fault Codes because it is within its pre-programmed range.

The same thing can happen the other way around and the sensor can "stick" at a LOW temperature reading which will have little to no effect on starting but will continue to dump far too much fuel into the system once the engine has warmed up causing a drop in MPG, increased detonation, premature spark plug failure, excessive coking of exhaust components and clogging of catalytic converters from the increased soot levels in the exhaust. This issue often manifests itself in poor idle quality, sluggish throttle response once the engine has warmed up, and excessive exhaust odor.

Gee thanks Mr. Wizard but how do I FIX it?

Check the electrical connection to the ECT sensor for corrosion and strength FIRST. Loose, corroded connections significantly affect resistance. I've posted the ECT and ACT sensor resistance chart several times in the past but here it is again. To test the ECT sensor, follow below and reference the chart.
With the engine OFF and COLD,
1) Take a resistance reading of the ECT sensor by disconnecting the sensor from the wiring harness at its connector.
2) Probe the two terminals in the sensor itself.
The resistance reading should most closely match the value shown for the temperature outside or a little colder since we are talking about a water-based liquid inside a steel enclosure. If the resistance is reasonably close to the corresponding temperature outside, the sensor is not stuck at a HIGH temperature. Continue to step 3. If the reading is off, the sensor is "STUCK HOT" and should be replaced.

3) Reconnect the ECT sensor, get the truck started and let it warm up fully. Shut it off and repeat steps 1 and 2. BE CAREFUL! EVERYTHING IS HOT NOW!
If the reading is reasonably close to the chart listing for 195-200º F the sensor is not stuck at a LOW temperature either and is most likely NOT the cause. If the reading is off, the sensor is "STUCK COLD" and should be replaced.

Apologies for the length... you all know I prefer to be thorough.

 

Thank you very much! I'll get this info over to the shop pronto!