I have a question for all of the ecm experts on here before I go and blow 100 dollars on a new ecm. Is it possible for a defective ecm to not allow the car to enter power enrichment mode? The reason I ask is my car is a big dog and feels as though it has no timing advance when accelerating. I monitored the O2 sensor and under heavy acceleration it shows 0 volts until I lift, I understand this indicates a lean condition. When the car is cold and in open mode it pings very bad when you get to about 3/4 throttle where you would expect PE to come into play. After the car warms up so that the knock sensor is operating approx. 140 degress the pinging goes away but I feel only because the computer is now retarding the timing. I have replaced every sensor on the thing, the last one being the MAF. Fuel pressure is good as well as the ignition components. Has anybody ever seen an ecm fail in this way before?
Thanks for any advice

 

Have you done any mods to the car? If so, what have you done? Is the PROM stock? How are you reading the O2 data, etc - with a scanner? What do the other sensors indicate? TPS readings should be about 4.5 volts at WOT. I think other members have encountered "dead spots" in a malfunctioning TPS that have caused similar symptoms.

PE mode (or WOT) is open loop. I don't think the O2 sensor comes into play at all, but I am no expert. Hopefully some others will chime in...

 

I'd guess you have a fuel delivery problem or the exhaust system is plugged up. Pressure may look ok, but you might want to disconnect the inlet and attach a hose. Place the other end in a coffee can and turn on the key. Should squirt about a pint. If not, try it with the filter removed. Otherwise, tape the pressure gage to the windshield and go for a drive (helps if you have someone observe the gage so that you can pay attention to the road). Assuming it checks out ok, unscrew the O2 and thread in your pressure gage (you might have to jury rig it with something from Home Depot). Anything over 3 psi means it's plugged up.

 

Contemplated your problem a bit more - make sure that the EGR valve isn't staying open. EGR when cold or at WOT will give the symptoms you described.

 

Thanks for the suggestions. I measured the voltage of the O2 sensor with a DMV at the computer, I realize that the O2 is not used in open loop mode but I was using it to gauge if the car was running rich or lean under heavy throttle, (I have read that it should go to .9 volts at WOT). I have chased down most of the things both of you have suggested and each time found problems but they did not correct the issue. I have verified fuel delivery by checking flow; and pressure both in the garage and while driving. The exhaust has a new front y pipe, no cats and a new main cat. The egr valve is a new AC delco unit, it had a cheap universal fit on there but I replaced that and the plastic vacuum control line thinking that was my problem. I have verified most of the other sensors to be working correctly, ie: MAT, CTS, TPS, MAF (new), knock, injectors. The reason I'm wondering if I have a bad ECM is it was replaced when I bought the car approx. 8 years ago to solve a problem that ended up being the fuel pump. I'm wondering if I got a bad part that gave me a new problem. There is one more thing that bothers me about this ecm, the egr temp sensor switch wire broke off of the switch about 2 years ago and I have not got around to replacing it but anyway it has never set a trouble code saying the egr is not functioning. I forgot to mention in the first post this is an stock 88 with stock prom. I'm sorry I made this so long it has just been driving me nuts.

 

It seems like you have covered quite a bit!

Sometimes a bad ground can wreak havoc with the electronics. Have you checked the negative battery cable for damage? What about where it bolts to the block (right around the oil filter)? There is also a ground strap that goes from that point to the frame. Make sure that everything is clean and fastened properly.

Does it bog down like that while revving it up in neutral too, or only under a load while driving?

 

I've covered the ground angle also, I purchased an electrical shop manual and cleaned and tightened all the ground points I could find. Not that it is a very good test but I verified the grounds for any resistance from the computer connector with a DVM. The car doesn't really bog the best way I can describe it is if you took an older 70's car and turned the timing back like 15 or 20 degrees it would run and idle but you could tell it is gutless. Here is another thing that is weird, I would expect that around town mileage for the car should be around 16 to 17.5 with heavy driving. The read out on the dash shows 19.6 and has been as high as 20. From what I understand the computer comes up with this number from injector open time. It doesn't seem like this system is that complicated to be causing this much of a problem but it really has me beat.

 

I am at a loss, too (though my knowledge is definitely limited).

Do you have a scanner (like an Auto X-Ray) or a laptop? A real time data log would be a tremendous help. You'd be able to see exactly what is happening as you go from idle to WOT. You could see if / how much timing is being pulled, knock counts, etc. at any given RPM. Has the ECM thrown any codes??

 

I don't have a scanner yet, I know that would probably be a big help. I do have a laptop, what kind of scanner or program do you recommend? I tried using that free program that you can download but it really didn't seem to show anything out of line. There are no trouble codes in the computer now.

 

I am using Craig Moates' ECM851 software. It is fairly simple and easy to use once you learn how to maneuver around it. It has a recording feature as well (which I'm sure most, if not all, programs do) that saves the data in an Excel spreadsheet. I like it a lot. What program were you using?

I also have an Auto X-Ray scanner, but it will only display data for one component at a time. I'll have to check, but I think I remember reading that it can only record up to 30 seconds of run time (though I could be wrong).

In my opinion, the laptop / scanning / data logging combination is far superior to the Auto X-Ray scanner.

There is certainly a possibility that your ECM is malfunctioning, but like you said - you don't want to replace it only to find out that wasn't the case. Do you still have your original ECM? If so, you could try swapping that back in to see if there is a difference.

 

Interesting that you're not getting a 32 with the temp/diagnostic switch disconnected. The switch grounds (closes) with heat and that signals the ecm that egr took place when commanded. You won't get the code if the harness is shorted to ground, the egr valve is stuck open (harness and switch of course would have to be intact and working or the harness shorted to ground), or the ecm is bad. A basic check of the ecm is to disconnect the TPS with it running. That should set a 22.

MPG and Range is calculated by fuel level signal, pulse width and a couple of other things. If the fuel signal is off, and that seems to be a problem as our Vette's age, so will the readouts. No readout at all is usually a bad ecm or an open in the serial data line between the dash and it's splice to the diagnostic link (which runs back to the ecm).

I don't know if a scan will tell you much, but you would want to look at Block Learn to see if it was rich or lean (idle and cruise) and then use the 44 or 45 Charts to check everything out. The Auto Xray will capture 14 seconds and if you buy the graphing software, you can transfer it to your PC and graph any 3 parameters. Unfortunately, it's capture rate is extremely slow, so finding intermittent problems can be difficult - though I don't think your problem is intermittent or happening so fast that you won't be able to capture it.

 

I down loaded the Craig Moates program, I think I will give it a try and see if I can find anything. Not getting a code 32 bothers me, other codes will trip like if I unplug the timing connector or something like the maf or tps senors. The egr switch wire is not shorted to ground the wire simply broke off of the top of the switch. Before the ECM was changed and again this was like 7 years ago a code 32 would trip in a heartbeat because of a bad egr valve. I really don't know why poor performance and no egr code would be linked together but that is all I can come up with.

 

Hopefully the software will tell you whether the switch is opened or closed (should be opened with it disconnected). In the interim, you could check for the signal at the ECM. Backprobe C15 for 12 volts. Jumper the harness to ground, voltage should drop to 0.

 

What is your fuel pressure under wot?

 

The fuel pressure is 38 to 40 at idle and 48 at WOT.

 

Ethan - did you get my PM? Any progress?

Dan

 

I did get it, thanks for the link, sorry I haven't got back with you work has been really busy the last few days. I think I'm going to try making the cable this weekend. I'm also going to pick up a new fuel pressure gauge and recheck the fuel pressure. One thing I forgot to mention is when I replaced the exhaust y pipe a few months ago the inside of the manifolds were super white like it runs very lean as I have suspected. Have you ever seen any kind of strange failures with the FPR? I will let you know as soon as I have numbers from the scan program. Thanks for your help.

Ethan

 

Glad to hear it. I know a lot of people on this forum have made their own cables, so if you run into trouble, help is out there (just not from me :o ).

FPRs do fail, but I've never had one go on me, so I can't describe the exact symptoms you'd experience. How do the plugs look? Are they bone white?

Hopefully the data logs will tell us something!

 

I haven't made much progress on the cable yet but the new fuel pressure gauge seams to have reveiled somthing. Maybe you have some insite. With the car running the pressure shows 38 psi and when you snap the throttle the pressure increases but the needle is very jumpy. When I unplugged the vacuum line to the fpr the pressure increased to about 48 but here is the bad thing when I snapped the throttle the pressure dipped back to almost 35. I haven't tried driving the car yet since the hose isn't long enough but it looks like there might be a fuel pump issue. The pressure seems to hold good at 45 when the car is off so none of the injectors are leaking. Any suggestions on a better test method of the fuel pump and regulator?

 

This is why I was asking what your fuel pressure was under WOT. If it's droping that much just under throttle snap, then imagine what it's doing under actual load! Duct tape that gauge to the windshield, and go for a drive.