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Just Some Dude in Jersey
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14,496 Posts
Discussion Starter #1
The stock 2.4 Motronic FI that I installed on my 2.4 engine works pretty good and is reliable, except for one thing: It hesitates or even stalls as I transition from idle......unless I do it SLOWLY. If I squash the pedal, forget about it, it will likely stall. It's almost excatly like the stumble that many Opelers with Webers encounter, except that fluttering the pedal, like when using a carb, doesn't aid in getting past the hesitation.

I'm thinking that it might be fuel pressure. When I ran some tests a few weeks ago in search of optimal fuel pressure for my set up, transition from idle was bad at 30-35psi, but seemed to get better with more pressure around 40.

Let's not focus on the pressure I chose for now. I'm wondering if eliminating the vacuum compensation would do the trick. Opel engines seem to be thirsty when transitioning off idle is often improved with more gas. The vacuum compensation on my system seems to drop the pressure 7-8 psi at idle, so that would make a pressure below 35psi. As you know, I'm using a returnless fuel injection system, the fuel rail dead ends and doesn't return the fuel to the tank. I'm wondering if the compensation is dropping the pressure too much. I'll have to check my thread on the subject to see why I chose to stop running tests after 40psi.

I guess I'm really starting this thread to ask generally: What can cause hesitation coming off idle with Motronic FI and what can I do about.

Tips, tricks, wild guesses welcome
 

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Just Some Dude in Jersey
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14,496 Posts
Discussion Starter #2
Here's the post from my Optimal AFR thread where I did the test on a hot day:

Gordo Tunes His Fuel Pressure
This morning I performed a series of tests at varying amounts of fuel pressure to determine which fuel pressure gave me the best air/fuel ratio and overall driving performance. I performed 5 test ranges at 35, 37.5, 40, 42.5, and 45psi. I didn't actually bother doing the last one for reasons that I'll reveal later.

To refresh your memory, I have a basically stock 2.4 with slightly bigger valves and the stock 2.4 Motronic Bosch 1.5 fuel injection system. I have modified it to use the Edelbrock returnless high pressure fuel pump system mounted in the engine compartment, with an Edelbrock low pressure pump to bring the fuel forward from the tank. I'm using an automatic on a separate tranny cooler. I have a 3-core copper radiator with an engine fan, plus an auxiliary electric fan on a thermostat set at 200* that turns on both radiator and tranny cooler fans. Both fans turned on while I was doing fuel pressure changes in various parking lots along the way. I have a heavily ventilated hood and lots of chrome. I use 20W50 oil and premium gasoline and the fuel selector chip for the computer is also set for premium.
Weather conditions were a real torture test for a GT, although not maximum bad. I went out at 10am and the temperature was 80 degrees with 80% humidity. 90+degrees is maximum bad around here with that level of humidity. A carbed normal GT would surely overheat in backed up traffic under these conditions. My temp gauge was reading 160-180, although I think it is under-reporting the temp by 10-20 degrees. I was well warmed up and the excursion took about an hour and a half, with all the stops to adjust pressure and jot down notes. I dealt with 5 miles of stop and go 20-25 mph traffic until I got to a country-ish road where I could get up to 50mph and try flooring it a bit. There were a few very gradual hills that most of you wouldn't even notice, otherwise the road was dead flat and straight.

Here's What I Discovered:
I thought I would be tuning for air/fuel ratio, seeking the safest A/F to assure engine health and longevity. I'm used to my Opel engines lasting forever(except for my recent 2). I don't give a fig about max power at 6000rpm, I drive at passenger car speeds and rpms, with only occasional bouts of spiritedness. My driving is about 25/75% highway and lower speeds.

Well, guess what? The computer seems to want to maintain an A/F of 13 at various 50 and below cruising speeds, no matter what the fuel pressure was. Idle was basically at 12.5 no matter what. Floored, the ratio went up to 14-14.5. It was interesting to observe the ratio wildly fluctuate for a few seconds, as speeds and load changed, while the computer compensated to try to get A/F to the vicinity of 13.
Since none of that stuff really changed, my results boiled down to how the driving performance changed at various pressures. Here are the results of that:

35psi - vacuum compensation at idle at 28 - Smooth economical driving, idle a bit faint, but steady, hesitation when floored.
37.5 - vacuum compensation at 30 - Nice driving, a bit more power, idle good, maybe a teensy bit of hesitation when floored
40psi - vacuum compensation at 32 - Nice driving, a tiny bit more power, no hesitation when floored, idle starting to get unstable when limping around at idle in Drive, computer trying to compensate, possibly on the verge of stalling.
42.5psi - vacuum compensation at 35 - Exhaust starting to sound loud and maybe rough with nice driving, power about the same as previous, maybe more, stalled twice while limping around, lots of idle compensation, A/F meter went off scale at 20(engine was pretty hot and weather temps were starting to get pretty unpleasant45psi - No point in trying this, I could see a negative trend developing, didn't want to stall in traffic.
45psi - No point in trying this, I could see a negative trend developing, didn't want to stall in traffic.


My Conclusion:
The computer keeps me at a safe, economical, A/F of 13.....my primary concern. I dialed down the fuel pressure to 38psi and liked the driving characteristics on this hot steamy day. I have to set my system to work harmoniously with my car's set up and the driving conditions in my area and my preferences. I could probably bump up the psi to 40 in cooler weather, but why bother, the difference in power would be negligable and I would probably unnecessarily waste a bit of gas. I don't normally drive my GT's in weather as hot and humid as today, but today's conditions were a good test of the upper limit of what my car can handle.
 

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Just Some Dude in Jersey
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Discussion Starter #4
This is the part that mentions hesitation:

35psi - vacuum compensation at idle at 28 - Smooth economical driving, idle a bit faint, but steady, hesitation when floored.
37.5 - vacuum compensation at 30 - Nice driving, a bit more power, idle good, maybe a teensy bit of hesitation when floored
40psi - vacuum compensation at 32 - Nice driving, a tiny bit more power, no hesitation when floored, idle starting to get unstable when limping around at idle in Drive, computer trying to compensate, possibly on the verge of stalling.
42.5psi - vacuum compensation at 35 - Exhaust starting to sound loud and maybe rough with nice driving, power about the same as previous, maybe more, stalled twice while limping around, lots of idle compensation, A/F meter went off scale at 20(engine was pretty hot and weather temps were starting to get pretty unpleasant.
45psi - No point in trying this, I could see a negative trend developing, didn't want to stall in traffic.


Just the fact that I included hesitation as one of my criteria let's you know that the hesitation thing has been a problem from the start. Keep in mind that the "hesitation" that I mention was when I was already off idle and trying to accelerate while already cruising. Maybe someone with more experience than me with stock FI can get clues from my test results.

Sure, I could decide not to wait for any replies to this thread and just plug off/disconnect the vacuum compensation and then go out and do some driving tests, but I thought I would ask if any other factors could cause hesitation coming off idle with stock Motronic FI.
 

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Opel Rallier since 1977
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1,848 Posts
Oooops.... I did not read the 1st post to see what was going on!

Vacuum compensation: Your Motronic computer was programmed assuming that the vacuum compensation is in place. The 7-8 psi pressure drop at idle that you cite is exactly what it should be doing. It is there to make the pressure across the injector, from injector fuel inlet to manifold pressure at the nozzle, to be a constant pressure difference so that the computer does not have to work around that variable. This vacuum compensation has been used on every factory FI system I have worked on (about 5 different ones), both for NA and turbo applications. So, I would not disable it.

You can block off the compensation but that is not fixing the problem. It may make it better but that is simply because it has enriched the mixture compared to where it would be with the vacuum compensation in place at less than WOT openings. Then the system will have to try to lean the mixture out at most operating settings for how you drive (idle to moderate vacuum levels where the fuel pressure is intended lower).

Airlfow sensing:
There are 2 areas used by the computer to judge engine airflow to work on that come to mind that cause this problem in other systems:
  • Bad or out of calibration TPS
  • AFM out of adjustment
The bad/out-of-calibration TPS is associated with the variable resistor on the side of the stock throttle body; tied to the throttle shaft. These going bad classically cause bad off-idle problems in all sorts of factory FI systems. And, with some older FI systems in the 80's, the computer did not have the ability to calibrate the TPS's actual position.

"Bad' means that the variable resistor (potentiometer) has rough or bad spots in the resistive element that the wiper moves across as the throttle rotates, and the voltage is not a smoothly changing voltage as the throttle rotates through its range. You can find the wiring diagrams and figure out which pin of 3 is that voltage. There will be ground, a fixed reference voltage (+5v for the Motronic 1.5), and the 3rd wire will be the potentiometer wiper voltage. That wiper voltage should vary smoothly high-to-low or low-to-high as the throttle is rotated through it range from closed (~0.6v) to open (~4.5v) for the Motronic 1.5. (There may also be some pins for WOT or closed throttle switches built into the TPS.) Seems like you have been through a lot of this part before....

Note that if you got a TPS from a C30SE, per the Motronic 1.5, the TPS rotates in the opposite direction of the other engines, and there are 2 wires in the harness swapped to accommodate this. The C30SE TPS rotates CCW.

Usually the only cure for a bad TPS is replacement. You can try cleaning inside with alcohol.

As for TPS calibration, if there are slots in the mounting flanges, then that says it needs position calibration. I've had an FI system like this, and the factory car manual had a procedure to calibrate the TPS position. IDK about this for your Motronic 1.5 setup, however, so I'll just mention it.


Now for the AFM: I've had the same off-idle hesitation problems with my Jetronic system and adjusting the AFM was the cure for that. (Those parts of the Jetronic and Motronic work similarly.) Rather than try to describe it, here is a video below that shows this adjustment. This was important in my Jetronic system to get rid of the off-idle lean condition. Reading the Motronic 1.5 operation description, it looks like the AFM is the most important element for airflow measurement in off-idle and mild throttle operation. So you may indeed have to try adjusting this.

If you do this adjustment, then:
  • Make sure you hold on to the toothed wheel when you release the lock screw; if it is quite free to move and you loosen that screw without restraining the wheel, then the wheel can rotate waaay off of calibration just from the spring tension.
  • Change this 1-2 teeth at a time: the looser the spring, the richer things will run.
  • And to get to this, you have to cut the glue holding the black cover on the AFM with a razor-thin knife.


Finally, FWIW: For the changes made on my Jetronic system:
  • The AFM adjustment has been used for fixing off-idle lean problems, both with stock and modified engine setups. I also found that the 2 AFM's I have were waaaay too lean with the stock engine, with some serious off-idle lean problems. I had to rotate the toothed wheel on both by over 3/4 of a turn from the settings as I found them to get things to work. This was done on 2 seprate AFM's; one had been messed with so just could have been set way off by someone before. IDK why the other needed so much change; it looked like it was unmolested.
  • Fuel pressure changes were only used to fix lean problems at large throttle openings after my 1.9L engine got larger valves and a mild cam with 2.25" exhaust.
Hope this all helps!
 

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Just Some Dude in Jersey
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Discussion Starter #7
Great advice and research! I just talked with Charlie and, alas, his video only pertains to the Jetronic AFM. He says that the Motronic AFM is "entirely different" and not easily adjustable inside. My AFM is a Chinese copy of one, so it is brand new. One has to take on faith that it has been adjusted correctly and is in good working order. So, that leaves the TPS as a possible faulty component. I would really need an analog volt meter to drops or blips in voltage, a digital one would likely give me dancing numbers. I tossed my analog meter. So, why not just get a new one? Gosh knows what the history of my present TPS is, it was randomly picked and "used, from Europe". I noticed that the piece inside, that rotates and slides over the throttle shaft, wiggles and slides around inside the housing a little bit. It struck me right off the bat: "Gee, if that's a potentiometer, that's the crappiest, sloppiest, one I've ever seen."

So, first I'll put a shiny new TPS and see if anything changes before I start monkeying around.
 

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Opel Rallier since 1977
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Starting with the sloppy TPS is a good first move. Just the voice of experience: The aftermarket TPS's can be pretty unreliable long term, so maybe you can find an NOS Bosch TPS.

But ya know... everything that I can find about the Motronic AFM on the 2.4L is that it is made the same way as the Jetronic, except for maybe omitting the fuel pump cut-off switch. Charlie's explanatory video shows the AFM being made the same way with it being the swinging vane type. And so does my document on the Motronic 1.5 system. (Which had the right info on the fuel pressures......) All that I see for this AFM as used in the 80's on the Porsche 944's etc, shows an adjustment inside.

Got a pix of your AFM? I am pretty sure you showed it before (mounted out front) and IIRC it is the swinging vane type. But I have not dug back to look at that pix to make sure....

As for taking it on faith.... I'll just tell you again that my 2 for the Jetronic were waaay off; perhaps they had been monkeyed with. (My viewing of Charlie's videos actually gave me the clue that the spring settings were way off.) Not sure why; the rest of the system was checked stem to stern and it was all correct except for the AFM setting. Unless it is for sure a direct lookup part, there is no way to know 100% if your Chinese knock-off would be right.... if it happens to be for a different make and model engine, then the spring can be very different as well as the setting. These springs were selected and adjusted, and the thick film potentiometer card potentiometer 'taper' selected and trimmed, so that the increase in airflow gave a specific voltage output vs airflow, and each engine design is different.

Your symptoms are classic for a leaning of the mixture, where the system does not see the right amount of air flow and supplies too little fuel. The TPS, and primarily the AFM, are what tell the computer about the airflow in this system. If the he fueling is OK at WOT is probably because the TPS is properly telling the computer that you are at WOT and it just automatically dumps in extra fuel at that time. Look at the 1st video I linked... it mentions these AFM's being consistently lean and producing excess NOx emissions.... that may have been for BMW's, but your symptoms are lean symptoms too. So the AFM is a prime suspect in my book.

You can experiment with the fuel pressure to see if the particular symptom improves as a way to confirm that it is going lean. But I don't believe that is where the real problem lies.

And there are some less common causes, like weak spark, that can cause these types of symptoms.
 

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Opel Rallier since 1977
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I dug in the pix and could not find a pix of your AFM. Was it explicitly a Bosch vane type and for the exact application from which the computer came?
 

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Just Some Dude in Jersey
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14,496 Posts
Discussion Starter #10
Pics of the new Chinese and a used Bosch Opel 2.4 Motronic AFM's. I installed the Chinese one.
The new TPS was ordered from Rock Auto and should arrive early next week. I'm going to install that before I do anything. :)

429574


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429576
 

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Opel Rallier since 1977
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Roger and tnx. Certainly looks 100% like the Bosch vane type AFM's. If the new AFM crosses over to the correct 2.4L Bosch AFM, you would think it would be right. No way to know if it is 100% adjusted right. Have you tried swapping in the older AFM and seeing if the behavior is the same?

One other question: Are the injectors new? If not, have the injectors been cleaned and flowed? There are folks who do that service. Just one more spot where the fuel control can be off.
 
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