Mystery hesitation

I have been stumped by this persistant hesitation for months. Attached is what i have tried w/o success. Any ideas?

This problem started in May, 2010 and has yet to be corrected, Any ideas?

Model 96 Chevy S10 PU, 4.3L, Automatic transmission, 4 WD ext cab

Notes 73k miles, one owner, well maintained


1 Occasionally difficult to start if left sitting idle for 5 days? will turn over w/o firing?did not fire when starting spray was used

2 Intermittant hesitation while driving during acceleration or just cruising?tachometer needle drops during hesitation

3 Have noticed that the locking torque converter fails to lock when the hesitation is particularly bad

4 Check engine light sometimes illuminates? code: Lean mixture

Measurements & trouble shooting

1 Fuel pressure measures ~ 42lbs

2 Sprayed water on the engine while running to see its effect on electrical system?no effect


Corrective actions applied but problem continues

1 Replaced distributor cap & rotor

2 Replaced spark plugs?no signs of fouling

3 Replaced HV ignition wires

4 Replaced air filter

5 Cleaned mass air flow sensor

6 Replaced inline fuel filter






It kind of sounds like an intermittent fuel pump problem. Try measuring the fuel pressure when it won’t start.

How old is the battery. I have a 2000 S-10 Blazer with the 4.3 V6. A few years back the interior lights were left on for a few hours with a fairly new battery. The Blazer started right up, but driving home I noticed the idle speed was a few hundred rpm higher and the torque converter would not lock up for the first 20 minutes of driving. There was no hesitation, but the truck seemed a little sluggish.

I checked the battery voltage, it was 12.1V (25% charged). I charged the battery back up to 12.6V (100% charged). The truck drove normally with a fully charged battery and has had no further symptoms since.

The first two batteries only lasted 2.5 to 3 years in the Blazer. Since the incident, I check the battery voltage about once a month and use a Battery Tender to charge the battery back to 12.6V as needed. The current battery is 4 years old and going strong.

Ed B.

If it was a fuel pump problem, why wouldn’t it fire right up with starting fluid?

True enough, I think that I got crossed (me, not the website) with another post that mentioned a lean mixture code.
This seems more like a crankshaft or camshaft sensor, but that’s just a wild guess too.

This really sounds like it might be electrical to me. If there is a sufficiently large intermittent electrical failure or a significant power drop it could be failing to spark AND pump. So you end up with a lean mixture that’s throwing the code but you also have spark issues which cause it to not start when you use the fluid. I’d do what edb suggests and start eyeing the battery first.

As an aside, the format of your question should be turned into a template for asking questions on here. We often get sort of snarky with people who just say “my car doesn’t work. Why?” so I figure it’s only fair to point out when someone posts a very nicely formed question.

More measurements and testing:

I tested the battery voltage with a digital voltmeter
Voltage measured 13.00v 2 minutes after turning off the ignition.
Waited 8 hours and repeated the voltage measurement…voltage measured 12.53v
Started engine, voltage measured 14.61v

The battery is 2 years old and cranks the engine over rapidly for over a minute.
Had the battery tested with load, it is good.

Alternator is 2 year old rebuilt. ALternator test showed it is good.

I’ll have to confirm fuel pressure test at idle and under way.

fuel pressure too low. minimum specs is 55 psi. far cry from 42 psi

I have done more fuel pressure testing tht may shed light on the health of the fuel pump:

  • Truck was parked for 5 days prior to taking these pressure readings. Pressure guage connected to the fuel line feeding the throttle body injector. Outside temperature 48 degrees F.
  1. Upon turning the key to the ON position, the pressure climbed to 50 psi, then dropped to 48 psi when the fuel pump relay de-energized.

  2. Cranked engine, pressure dropped to 45 psi, engine started up. Fuel pressure read 48 psi at idle,

  3. During test drive, the pressure increased to 58 psi under hard acceleration.

  4. Cruising at 50 mph, the pressure read 55 psi.

  5. With the truck coasting, the pressure reads 45 psi.

I don’t know the specs for the fuel pump, but judging from the feedback it seems that the pressure may be borderline. Any comments are welcome.

The injectors might be worth looking at. The no start with starting fluid is the reason for that leap of faith. If the cylinder is flooded due to a leaking injector that could describe all the symptoms.

under hard accelleration you should see 66 psi. if not, you need to find out why. Could be leaking fuel pressure regulator, but I would have to believe it would be running pig rich because the regulator is mounted in the intake. By the way, the exact specs are : key on engine off= 60-66, idle 50-63, wide open 66. Kinda low on the miles for fuel pump replacement. Have you ever ran out of gas? That will definitely shorten the life of a fuel pump. If I were you i would try another gauge to make absolutely sure. You will also want to confirm that you have good voltage going to the pump and also has the ability to carry the amperage. To test this I would uus an old style headlight so when you turn it on it will load the circuit, which a test light cant do. If you confirm voltage and amperage, time to pull the trigger on the fuel pump. Good luck!

  • I checked the pressure guage with a second unit and the two units read the same pressure.
  • Since the fuel pump is capable of providing 58 psi pressure under load, it seems that the fuel pressure regulator is determing the pressure that the injector recieves from the pump. The regulator cannot increase the pump’s pressure, but can only lower the pressure by openning to allow the fuel to recirculate back to the tank.
  • The spark plugs showed no sign of a rich fuel condition but fuel mileage does seem to have dropped. An test of fuel mileage is being conducted.
  • Is the fuel pressure regulator controlled by the onboard computer? Can I test it’s function.

with the way these pump modules are built I dont really think you want to deadhead it or you run the risk of blowing apart the line inside the tank from the pump to pipe. There is a spot where you can use a pair of pliers to momentarily restrict flow. Youll see the braided lines underneath. While the engine is running, pinch the small line to momentarily restrict fuel flow back to the tank. Have an assistant watch the fuel pressure to see if it rises to specs. If it does, then you’re right its the regulator. If it stays the same, go to the pump. If it is the reg, its located in the intake. If youre gonna tackle this yourself get a manual. Youll need it.

Inspect the cam angle sensor. It may be getting sludged up and dropping its signal. But then, the relucter(magnet) in the cam gear may be getting loaded with sludge and metal dust causing a weak signal to the sensor.

Updated January 1st, 2011.

The hestiation issue has not occured for over a month but the starting issue has worsened.

The truck does not start easily if left sitting idle for extended periods of time.

The truck starts easily if it has benn sitting for only 4 - 6 hours.
When sitting overnight it will have to crank over several for several times before actually firing.
If left for several days, it will have to crank for ? 1 minute and then will start and run rough for 15 seconds.
If left for 5 days it will crank for minutes and may not start at all. The battery will evetually require charging and then it will eventually start.