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Runs fine for about 10 min, then shuts down under load
I did some searching and haven't been able to find a similar issue. The car is new to me and I've been trying to learn about it and see if I can get it running better than when I got it. I have an odd problem with it. It runs fine driving it around the neighborhood then after about 10 min it basically won't move under load. It idles, but then the minute you let the clutch out it basically feels like it shuts off, push the clutch back in and it goes back to idling. If I shut the car off and immediately restart it, then it seems to go back to running fine. It feels like the ignition is cutting out under load, or it is running out of fuel, but basically the minute you apply load by releasing the clutch there is basically nothing. Wanted to get your thoughts. Are there any things that I can look for diagnostically when it happens i.e. any data I can get from the PFC that might indicate the source of the issue?
Some details about the car:
Compression is on the lower end 74psi on all rotor faces - I plan to rebuild it as part of the restoration, but want to see if i can sort out as many issues as possible, so that when it is rebuilt I'm not dealing with legacy issues.
High flow intake, Trust intercooler, down pipe, mid pipe hks cat back.
I changed the fuel filter and checked the fuel pressure ~30-40 psi. Couldn't see gauge when the issue occurred, but after, when i got back to the garage it was still showing in that range
I get boost in the 5-10 psi range.
It had stock ECU when I bought it, just switched over to PFC and did a FCTweak first pass on it. Problem existed before with stock ECU as well.
You got some weird numbers but I would say start with the water temp sensor. The voltage is a little odd for the given conditions. Replace it and we can go from there if there's no change.
Thanks, I'll do that. Interesting thing today, when I loaded the log file into FCTweak it gave me a water temp sensor fault as well, but wasn't sure if that was from starting the log before it was fully warmed up.
Greg
ayyy nice lol it likely sees what i'm seeing. given the conditions, the water temp voltage doesn't correlate with everything else. could also be the wiring at the plug honestly but its not uncommon for the sensor itself to fail. normally it wont read at all though. post a picture of the plug if you can.
Connector tab is busted off. Looks like when PO changed the alternator they probably busted the connector. Maybe they damaged the sensor as well if they flexed it to much. Got a new sensor on the way, so will change it out and see what happens.
Thanks FD Auto, new coolant sensor seems to have resolved that issue. Now I can drive without it cutting out and stumbling after 5 minutes of driving. Now on to the next problem. It runs great until I get into the boost and then it cuts out and starts backfiring out the exhaust. In first and 2nd gear I can maybe get to 2 psi before it happens, in 3rd gear I can get to about 5psi before it happens. I haven't done any searching yet, but if anyone already knows the answer I'm happy to look into it :-)
Greg
for this new issue, try going under settings and turning o2 feedback off. see if that solves it.
if it doesnt then youll make sure your tps is swinging correctly. to test it you will put the key on not running, go to the sensor check screen, watch vta1 and vta2, slowly and gradually press the throttle. you should see the voltages rise gradually and not jump around.
closed throttle
vta 1: 0.10-0.70
vta 2: 0.75-1.25
fully open throttle
vta 1: 4.20-4.60
vta 2: 4.80-5.00
vta 2 should hit ~5v before fully depressing the pedal. this is basically the tps calibration process. presumably your cold idle functions are fully intact because of the high vta voltage during cold idle. also make sure the map sensor setting is correct. under setting and pim volt, you should have option 1 selected. using the commander like this can make troubleshooting pretty easy. we'll just go through the steps and we should be able to pretty easily figure this out.
throttle position sensor looks good at open and closed and looks to climb smoothly as you depress throttle. VTA2 maxes out at 4.97v about half throttle.
cool... we can rule out standard sensors now. now we will move on to other possibilities. pull up injector duty, boost, rpm and air temp on the commander. go for a drive and get it to sputter and do what its doing. make sure the peak setting is enabled. i think its up or right on the d-pad and the peaks will show up next to each parameter. post a picture of that screen
Here are two logs of peak values. As a side note I was getting similar symptoms to what I had prior to changing the coolant sensor on the first drive. Fuel was below 1/4 tank, so I put 3 gallons in it and it seemed to be back to normal with regards to off boost driving. So not sure if is was a fuel issue or if something else weird is going on. The two logs are after adding fuel and just cutting out when boost kicks in. Also first two or 3 pulls it pulled really strong in 2nd gear and seem to boost fine.
fuel pressure is likely fine since your injector duty looks good. your air temp is extremely high. what i bet is happening is you're leaning out or your secondaries are acting up. there is an air temp correction map but walking you through that on forum isn't practical. do you have a wideband installed? to check the secondaries would require accessing them and then you enter a realm with endless possibilities lol
Thanks again, I do have a wide band installed and can datalog it. The current tune I have on it is done with a first pass using fctweak, then one run using the AI tool. After replacing the cooling sensor as it fctweak also noted it was providing irregular data, I also got a high knock on the next pass with it. I have a fuel pressure gauge hooked up, but can’t see it while driving, so will see if I can point a GoPro at it just for additional data. I’ll log some data tomorrow and report back.
Did some runs this morning. When boost starts to kick in AFR immediately spikes super lean. Also captured a video of the fuel pressure and it normally runs about 35-40 psi and then when boost kicks in and it starts to stumble it bounces between 10 and 20 psi. Have about a 1/2 tank of fuel now to eliminate any starvation issues.
Reading up on fuel pressure, it looks like it should basically increase over the base pressure reading by the amount of boost as boost increases. It looks like that is controlled by a signal from the ECU to switch the vacuum solenoid and apply vacuum to the regulator to increase pressure. Assuming that is correct does the FPR indicator on the power FC indicate when the solenoid should be switching on to add vacuum to the FPR? Just wanted to verify if that indicator changes state it is likely signaling for more pressure assuming that is what it means. Then I guess next step would be to check actual solenoid operation and check vacuum lines. I can see with a mirror that the vacuum line is connected to the FPR, but everything is old and crusty under there, so lines could easily be broken or popped off somewhere else. Any checks I can do before pulling the UIM?
I pulled the UIM and checked the vacuum lines to the FPR and they all seemed intact, hard to see where the one connected to the vacuum source side goes, but felt like it was still connected and felt pliable. When I was 90% done with removing the UIM I noticed the boost sensor vacuum line was off the sensor, but think it got knocked off when removing the pipe to the throttle body. After reassembly it initially felt pretty good and seemed to boost a little without the afr going lean, but then went right back to doing it after I drove about 5 more minutes where I could get into it a little more. I also verified boost sensor line was still on after the drive. Here is a better video of the fuel pressure. From this drive.
Quick update on what I’ve done today. Someone suggested checking the fuel pump speed relay. I took it out and bench tested it and it was not clicking, then pulled the cover off to have a look inside and there was nothing obvious. Retested it with the cover off and it started working maybe it was stuck and some fiddling broke it free, who knows. I also found another relay in the box of parts that came with the car and it tested good, so installed that and went for a drive, no improvement fuel pressure still dropping to 10 psi when boost comes on. I decided to look at the fuel pump resistor and noticed the wire on its pigtail was mostly bare in one spot, so pulled it out. When doing that I also noticed that where the main harness wires for the resistor pass through into the wheel well there was electrical tape, pulled that off and found a section about 1 1/2” long with no insulation and some damaged wire strands. I cut that out and re-spliced it back together. Also just to take the high speed relay out of the equation I cut the connector off the old resistor and jumpered the wires together to eliminate the resistor and give it full voltage all the time. I hooked up the volt meter to the fuel pump connector in the hatch and was getting 13.5V. Went for a drive and for the first time it ran cleanly all the way up to 10psi boost going from 1st to 3rd gear. And voltage stayed at 13.5 the whole time. Then did a 2nd pull after going around the neighborhood again and it went back to dropping fuel pressure like it was before. The fuel pump looks to be original, so ordered a new one just to see if that fixes it, my only thinking is that maybe it is just weak and some more voltage was enough to give it a temporary boost for a few minutes. Don’t like throwing parts at it but running out of ideas.
Seems in this case your injector duty did not do what it normally does in the event of low fuel pressure. If you have the stock pump with the o ring fitting on top you will want to "upgrade" that to a section of fuel hose instead. We have a how to video on our YouTube channel.
The wiring issues definitely aren't helping the situation so addressing that will be good as well. For a quick test/fix, you can replace the o ring fitting on the hanger just to see if its consistent. Max voltage on the pump gave you a good result temporarily so maybe it was enough to kind of bypass the failed o ring until it basically failed again.
Check out the fuel pump video on our YouTube. If youre going into the tank to do work, then it would be worthwhile to do a complete upgrade
Thanks for the feedback. I pulled the pump out and the o-ring looks to be intact and pliable, but could be leaking due to age etc.. I have a new pump coming, so will install that and convert over to the submersible fuel line connection setup. Great video!
Greg
Got the new pump and installed it today. That solved my problem, now I’m getting about 52 lbs of fuel pressure somewhere around 10-12 lbs of boost. The needle on the fuel gauge doesn’t dip when going on boost and continues to climb as boost increases and my AFR is looking good now. Before pressure was dropping to 10psi with about 5lbs of boost. Of course I pulled the old pump apart. The impeller and housing looked really good. The commutator had quite a bit of wear, but the brushes were still making contact, though it wasn’t contact across the full face of the brush. They also looked like they had plenty of material left. I suspect that they just weren’t making good contact and under load the motor couldn’t keep up. It is possible the o-ring on the pump where it connects to the fuel hanger was leaking, but I’m leaning towards the above as the o-ring looked good and was pliable. Some internal pics of the pump for reference. I broke the impeller getting it out, was intact when I opened it up.
08-06-23, 04:56 PM
