Intermittent rough running while accelerating

WildSide · Mar 23, 2016

Hi guys, I have an 88 2.9 5 speed, this engine has begun to act like it is dropping cylinders sometimes while accelerating under light or partial throttle when warm, strangely this has happened when making a right turn around a bend as well, when running steady at highway speed she smooths out and runs fine. No codes and fuel pressure is 40 psi. Has anyone experienced this?

RonD · Mar 23, 2016

I would first check fuel pressure, raise RPMs about 2,000 and hold it there for a minute or 2, see if it is dropping.

And I would do the test with 1/4 full gas tank or less.
When was fuel filter past changed, under $15 to do that.

'88 should have 2 fuel pumps, the one in the gas tank, pick up pump, and the one in the frame rail below drivers seat, high pressure pump.

The going around a corner stalling reads like you may have a cracked line/hose inside the fuel tank, this lowers fuel pressure when accelerating because demand for fuel goes up, but when exposed to air, gas sloshing to one side, pressure drops quickly then recovers.

Other possibility is failing TFI module, spark timing needs to change when accelerating, I would expect just sluggishness if it was spark timing related not misfires through, and wouldn't explain the turning misfires?

WildSide · Mar 23, 2016

RonD said:
I would first check fuel pressure, raise RPMs about 2,000 and hold it there for a minute or 2, see if it is dropping.

And I would do the test with 1/4 full gas tank or less.
When was fuel filter past changed, under $15 to do that.

'88 should have 2 fuel pumps, the one in the gas tank, pick up pump, and the one in the frame rail below drivers seat, high pressure pump.

The going around a corner stalling reads like you may have a cracked line/hose inside the fuel tank, this lowers fuel pressure when accelerating because demand for fuel goes up, but when exposed to air, gas sloshing to one side, pressure drops quickly then recovers.

Other possibility is failing TFI module, spark timing needs to change when accelerating, I would expect just sluggishness if it was spark timing related not misfires through, and wouldn't explain the turning misfires?

I did consider a cracked line or perhaps a short, I just remembered last night the tank gauge dropped to empty and the engine ran silky smooth, when it started registering again the roughness/stumbling returned.

RonD · Mar 23, 2016

Definitely sounds electrical in that symptom.

There should be 4 wires under the truck that go to the gas tank

pink/black stripe is the fuel pump power, it is spliced near the high pressure pump, so both pumps get power from the same wire.
Black wire is the in tank fuel pump Ground, high pressure pump is usually Grounded to the frame near its location, but can be spliced into Black wire.

Yellow/white strip wire is from the dash board fuel gauge and runs to Sender in gas tank
Orange wire is the Ground for the Sender in the tank.
The fuel gauge gets 5 volts from power supply in the dash, the Sender is the Ground for the gauge, on an '88 when tank is empty the Ground resistance is high(high OHMs), so disconnected Ground would show as Empty on gauge.
Grounding the Yellow wire would show Full on the gauge.

1989 and up is opposite.

So gauge dropping to empty could mean a bad ground on the circuit.

WildSide · Mar 23, 2016

Thanks Ron I really appreciate the help, I did not get to the electrical yet, I did check the engine in more detail and found the throttle linkage to be very sloppy which is not good, it had a large bore TB now replaced with a good small bore,the engine starts right up and is smother but I need to check the TPS adjustment and see if the acceleration problem continues when the engine is at temperature, right now there is no problem when cold. Strange but I discovered a small piece of rubber tubing inside the IAC inlet restricting air flow.

RonD said:
Definitely sounds electrical in that symptom.

There should be 4 wires under the truck that go to the gas tank

pink/black stripe is the fuel pump power, it is spliced near the high pressure pump, so both pumps get power from the same wire.
Black wire is the in tank fuel pump Ground, high pressure pump is usually Grounded to the frame near its location, but can be spliced into Black wire.

Yellow/white strip wire is from the dash board fuel gauge and runs to Sender in gas tank
Orange wire is the Ground for the Sender in the tank.
The fuel gauge gets 5 volts from power supply in the dash, the Sender is the Ground for the gauge, on an '88 when tank is empty the Ground resistance is high(high OHMs), so disconnected Ground would show as Empty on gauge.
Grounding the Yellow wire would show Full on the gauge.

1989 and up is opposite.

So gauge dropping to empty could mean a bad ground on the circuit.

RonD · Mar 23, 2016

And just as a heads up.
Alternator starting to fail can limit voltage to the whole system, which can also cause misfires when engine is under load.

Before starting battery voltage should be 12.3v to 12.8v
After starting battery voltage should be above 14v but below 15volts
After driving or idling for 5 minutes, battery voltage should be 13.5v approx.
Turn on all lights and heater fan to High, battery voltage should drop and then come back up to 13.5v
If you increase RPMs above 1,500 voltage should go up a bit and then settle back down to 13.5v

WildSide · Mar 23, 2016

Yes power output is very important including grounding. I had to adjust the throttle to bring in into specs and the engine is behaving so much better so far for a short run test, not a misfire at all and she is running smooth. Not to say all issues are solved! I am looking at every small detail.

WildSide · Mar 24, 2016

I did recheck alternator output and battery voltage

Battery 12.5V
Alternator 14.73 V
Running V 13.5
While the engine is running much better I am checking all grounds and voltage at the pumps.

RonD · Mar 24, 2016

Thanks for posting the update

gw33gp · Mar 24, 2016

Another thing to check is the spark plug wires. I replaced the spark plug wired on my Ranger a few years ago and went out for a test drive. It was running perfect until I tried to accelerate hard. It started missing until I went back to cruising. I checked everything out and it seemed fine but pushing on each boot at the spark plug revealed one had not full clicked in place. Once all the boots where seated, no problem.

I had a similar problem with a 99 Cougar about six months after replacing the spark plug wires. All the boots where seated properly but it turned out to be one of the boots started allowing allowed tracking (arcing along the porcelain insulator) resulting in a less than adequate spark at the spark plug. Cleaning the inside of the boot and a new spark plug solved that problem.

I also have a 69 Cobra that likes to unseat a spark plug boot occasionally. Again, it will run well until I accelerate hard. Once I seat the boot again, no problem. I finally siliconed that boot in place and resolved the recurrence of that problem.

There are many things that can happen in the spark plug/spark plug wire area. Make sure they are good and are working properly.

WildSide · Mar 24, 2016

Wires and plugs are good, I check resistance on the wires and make sure the terminals are snug. I did have a set of E3 plugs once, it turned out one plug was defective causing acceleration problems, crappy customer service led me to donate those plugs to the garbage dump.

WildSide · Apr 7, 2016

I thought I would give a final update on this problem as it has now been fixed, after a run of aprox 30 KM the problem reappeared along with stalling while decelerating. After another TB adjustment higher the 2.9 is running smooth as silk. I swear this problem seemed like it was fuel pressure related! Lesson learned.

RonD · Apr 7, 2016

Thanks for the update

Fingers crossed

Intermittent rough running while accelerating

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