Odd Acceleration Issue & Occasional Pulsing Idle (1 Viewer)

[SmokingRocks]

Recently I've noticed an interesting issue that seems to randomly happen in the morning when the rig is cold.

When setting off for the first time I'll roll about 30% into the throttle and the rig is SLOW to accelerate. So I add more throttle and the power comes on like a light switch. It seems to only happen once then it's fine.

No codes.

It hasn't become such a major issue that I've needed to put the rig out of service to figure it out, so I'm casually troubleshooting what the issue could be. My first thought is the fuel pump relay is not working right, since it feels like the engine is only giving up to 50% power, then press further into the gas pedal and BAM 100% of power becomes available.

But since this only happens once and it's fine for the rest of the day I'm thinking it's not the FPR (also the FRP is less than 2 years old), but perhaps the signal it's getting. And if that's the case then what's the real cause? The ECU? or one of the sensors that it's monitoring like the throttle position sensor?

Anyway, I just wanted to float this out there and see if anyone has experienced this before. I guess my next step is to wire up a voltmeter to Port 1 of the Data Link Connector (fuel pump +) to see if that's the system having an issue.


I've also had a very loopy idle randomly if I let the rig idle for more than a min or two once warmed up, but I think this was an EGR issue and since I've disabled it I haven't experienced it again. Idle is super lumpy when cold, very similar to a major vacuum leak

 

[SmokingRocks]

Now that I'm back from my trip and since this is still an issue I've decided to get out my new O-Scope (Pico Scope 2204a) to start testing things.

1st Test

• Chanel A: Fuel Pump Voltage
• Channel B: Fuel Pump Relay Trigger
• Expectations
  • Channel A should be at reduced voltage ~8v until the ECU triggers the Fuel Pump Relay at which point the voltage should rise to ~13-14v
  • Channel B: at idle and low throttle positions should not show continuity with ground, under higher throttle inputs the ECU should trigger and we should see channel b drop to ground (continuity with ground)

Heres the EWD with probe locations noted

Test 1 Results

So the relay is switching correctly however I'm confused by the signal channel B is picking up for the fuel pump relay trigger. I'll have to check to see if the pin is in all the way and on the right port. I expected to see channel B drop to ground when channel A rises to 13v, but instead it seems like we are looking at one of the injector's waveform. This could also be pointing to another issue with the injector wiring, which I've already dealt with previously and can be seen in this thread: Calling all you Diagnostic Gurus | SOLVED Stumbling on hard acceleration

Update: Turns out I pinned 11A which is the injector for Cyl2 both are the same color wires... I'll need to re-run the test with the correct pin.

 

[Racefiend]

Yeah it looks like you're on the wrong pin at the ECU. You should see 12v with the relay off and ground with it on. But the fact the voltage at the pump changes tells you the relay is working. The Pico is great for checking sensors. Check your oxygen sensor for proper switching, and check your TPS for smooth voltage change with no dropouts. Log the VF voltage at the check connector as well. That will tell you your fuel trims. Also check your coolant temp sensor voltage during a warm up.

 

[SmokingRocks]

Thanks for the feedback, the Pico is certainly a step up from my homemade scope that I plugged into my iPad. I'm loving the pico.

And yea, I was thinking of checking the TPS, Coolant Temp Sensor, RPM Signal from the distributor and basically all the sensors the ECU is looking at to control the engine. The O2 sensors are a good suggestion, I'd hope they are good as they are upgraded Bosch units less than 2 years & 20k miles old.

 

[elripster]

I will be watching this one closely. I had a similar issue and I never figured it out. Just recently it came back but this time it through 25 and 26 (lean and rich at the same time because why not?...) and I have one O2 sensor reading in very low. I will replace this weekend and retest.

Frank

 

[White Stripe]

How long has the problem been happening for?

 

[elripster]

How long has the problem been happening for?

For me or Smokingrocks?

 

[SmokingRocks]

It's been happening for a while. It's intermittent which makes it harder to track.

I think the surging idle and the delayed triggering of the FPR are related but they may not be. Upon cold startup, the engine will idle fine for a minute but as it gets warmed up it will begin surging and the surging gets more and more drastic. I guess this points to the IAC getting things wrong.

 

[elripster]

Well, at start up the engine is running open loop so ignoring sensors and running on known good calibrations. The O2's heat up within a minute so I'd start by measuring their voltages. The low power/surging to me felt like the truck was running out of fuel. However, like you mention, I could floor it and it would take off indicating the fuel system was capable of delivering plenty of fuel if it was asked to do so. If you have something similar then possibly the issue is that it's not being asked by the ECU. If that's the case then an out of range but not far enough to trigger a code sensor could be the culprit vs. an actuator.

Frank

 

[elripster]

If a new O2 sensor doesn't fix my truck I am getting a Pico ASAP. The thing that was frustrating is not having realtime data like one would have on an OBDII vehicle via CAN.

Frank

 

[White Stripe]

It's been happening for a while. It's intermittent which makes it harder to track.

I think the surging idle and the delayed triggering of the FPR are related but they may not be. Upon cold startup, the engine will idle fine for a minute but as it gets warmed up it will begin surging and the surging gets more and more drastic. I guess this points to the IAC getting things wrong.

I'm not sure I'm following exactly all the details. But from what details I think your telling I think the afm is probably your power issue. The idle problem is probably the iac. Mot saying those are for sure your problem. That's where I would look. Also for a pre 95 I highly recommend installing a air fuel ratio gauge permanently. Helps give more data of whats going on under the hood.

 

[White Stripe]

For me or Smokingrocks?

Smocking rocks. Your problem to me sounds like the o2 wiring plug has corrosion and or a afm that is getting worn. You can try to adjust them. I've done it with some success and failure in the past.

 

[elripster]

I have measured my system to the nth degree. I have a whole saga posted up on it, it's one big TLDR but the punch line is that everything measured in spec and still the truck did what he describes here. Only last week did start the symptoms again and also through codes. Before it was running horribly with zero codes.

I just impulse bought a scope. My buddy needs help troubleshooting wheel speeds and it's what one really needs here to see what values flip to what when phenomenon come/go.

I put together an excel sheet where you can enter values it turns red/green if in/out of range. I'll post it up when I get home if it helps.



Frank

 

[SmokingRocks]

So I feel like I've done a terrible job describing the issues, so let me break them down separately.

Sluggish 1st Acceleration
When I pull away from my house in the morning (first drive of the day) I lean into the gas and the engine is underpowered, then I lean in even further and suddenly it turns on. I've now confirmed that this is due to something delaying the ECU triggering the FPR into "high flow" state. Once I trigger the high flow state on the FPR the problem doesn't reoccur, usually staying dormant until the next cold start first drive.

Surging Idle
On 75% of the cold starts (and sometimes after warm starts), the engine will initially idle fine, then a surging (revving up and down from 700rpm to 250 or less) will begin. The surging seems to increase in severity as time goes on, perhaps indicating an IAC valve issue as it could be trying to chase the correct idle.

Again NO codes are present.

I tested the AFM less than 6 months ago and it was within the specs listed in the FSM. The O2 sensors are the bosch upgraded ones and are mounted in the pair holes on the upper exhaust manifold. I've disabled the EGR by removing the Vacuum references to it. I guess the next easy test is to unplug the IAC valve when the surging is happening to see if that changes anything.

The rig drives just fine, had enough power to make the trip over the colorado mountains to moab at 75mph last week and return 12mpg. So the issue isn't affecting drivability or economy.

 

[White Stripe]

I have measured my system to the nth degree. I have a whole saga posted up on it, it's one big TLDR but the punch line is that everything measured in spec and still the truck did what he describes here. Only last week did start the symptoms again and also through codes. Before it was running horribly with zero codes.

I just impulse bought a scope. My buddy needs help troubleshooting wheel speeds and it's what one really needs here to see what values flip to what when phenomenon come/go.

I put together an excel sheet where you can enter values it turns red/green if in/out of range. I'll post it up when I get home if it helps.



Frank

If it's corrosion in a plug it may ohm out ok at certain temperatures or moisture levels. But not at other temps or when its dry.

 

[SmokingRocks]

I'm not sure I'm following exactly all the details. But from what details I think your telling I think the afm is probably your power issue. The idle problem is probably the iac. Mot saying those are for sure your problem. That's where I would look. Also for a pre 95 I highly recommend installing a air fuel ratio gauge permanently. Helps give more data of whats going on under the hood.

Wide band? which bank do you suggest monitoring?

If it's corrosion in a plug it may ohm out ok at certain temperatures or moisture levels. But not at other temps or when its dry.

easy enough to unplug and clean, I'll do that tonight.

 

[White Stripe]

Wide band? which bank do you suggest monitoring?

easy enough to unplug and clean, I'll do that tonight.

For the corrosion, that was for @elripster. Maybe you should start your own thread elripster. It's getting too confusing.

The corrosion i was talking about for elripster doesn't sound like your problem Mr. Rocks, at least to me. As far as the wideband I'm not real familiar with the stock early model exhaust. Do both banks tie in before the cats or after?

 

[SmokingRocks]

If I had more information on which sensors the ECU is evaluating to determine when to trigger the FPR I could narrow down my search for the delayed acceleration issue. But this is all the FSM gives me

Both banks tie in after the cats, I mean I can scope the O2 sensors and determine the A/F Ratio based on voltage seen, since Lambda is 450mv which = 14.7:1 A/F Ratio.

 

[elripster]

I already have a thread, it's huge, went on for months and ended up with the truck miraculously fixing itself. This is why I am interested to see how this turns out since this truck is exhibiting similar symptoms.

I'd definitely measure the IAC, it's an easy test in the FSM. If it checks outs consider that the ISC essentially closed loop control of a stepper motor that moves a rubber plunger in/out. Yours, like mine, would grow more unstable as the truck hunted for a position that would produce a smooth idle. For example, it attempted to reduce the speed, but the response was too severe so it over corrected due to its PID not being tuned for whatever conditions are present thus growing more unstable. It could be, for example, under fueling at idle, or something else that has changed the conditions for which it's designed to operate. That the truck is actively trying to control its idle means the stepper is most likely working. I'd remove the ISC and check the condition of that rubber plunger as well as spray some carb cleaner into the ports to make sure they aren't plugged.

If the actuators check out then this is probably a sensor side problem. Maybe the O2's glitch at one part of operation and the ECU has learned some corrections that are hampering idle/low load performance. That scope logging data while driver could be key.


Frank

 

[elripster]

If I had more information on which sensors the ECU is evaluating to determine when to trigger the FPR I could narrow down my search for the delayed acceleration issue. But this is all the FSM gives me
View attachment 2974547

Both banks tie in after the cats, I mean I can scope the O2 sensors and determine the A/F Ratio based on voltage seen, since Lambda is 450mv which = 14.7:1 A/F Ratio.

Even a tiny movement of the AFM triggers the FP. Key on, move the AFM flapper, you should hear the hiss of fuel commence. If you install a fuel pressure gauge you should have the requisite PSI. If that hiss doesn't occur, the truck won't run. The FP itself is Boolean on/off with pressure controlled by the mechanical regulator at the forward part of the fuel rail. There is some vacuum modulation of the fuel pressure though I forget exactly how that changes it. If you start the truck, you'll see the flapper door move quite a bit and it will pulse with the change in idle as it should.

Note, if you do start the truck and the idle is surging, try manually moving the AFM door inward some. If the idle smoothens, at least you know you have a lean condition and we can work from there. The AFM is also easily testable and so I'd do that as well just to rule it out.

Frank