Missing at high rpm and WOT

#1

Here’s a lesson learned…My car was missing at high rpm and WOT really badly. It was pretty much ok at high rpm and partial throttle, and it was pretty much ok at mid rpm and WOT. Only the combo of high rpm and WOT showed the severe symptoms.

I have a fuel pressure gauge on my dash so I knew the problem was not fuel.

It turned out that my alternator was a bit loose such it could rock forward and back. Not such that the belt could loosen. I think that this was affecting the ground of the alternator. Once I tightened the bolt under the alternator, most of the high rpm WOT missing was fixed.

#2

We ended up having to run a dedicated alternator ground from the alternator body to the chassis on our 24hrs of LeMons E30. That fixed some of the stupid electrical crap we went through … some of it :slight_smile:

#3

This was an interesting problem because, near as I can figure, it indicates that the DME becomes especially intolerant of system voltage problems when the TPS tells it to go to internal maps.

The charging V Bentley spec is 13.5 to 14.2 IIRC. My charging V is 13.75. That means that the consequence of my alternator moving around a little was system V varying between 12.6 from the battery to 13.75V from the alternator. And somewhere between those two voltages is some threshold that the DME at WOT sees as a brownout.

You’d think that 12.6V would be adequate to run the DME and the ignition system, but apparently not under all conditions.

There is still a little bit of missing at high rpm tho. There is some other problem that causing a similar symptom, if not as severe. It’s bad enough tho that the car doesn’t hardly accelerate in 4th gear >5k rpm.

After talking to various smart folks about this, here’s the action plan…

  1. Remove connector to fuel injector harness and hard wire. Done.

  2. Swap out batteries and see if charging voltage nears the top of the range. That is to say goes from 13.75v to something approaching 14.2V.

  3. Install a new voltage regulator. Ordered.

  4. If none of that does the trick, swap in a spare alternator.

The thread below talks in more detail as to why I got wrapped around system voltage as a possible culprit, even tho it’s within the Bentley spec. The bottom line is that just because your shit is within spec, doesn’t make it good.

http://www.e30tech.com/forum/showthread.php?p=1217789#post1217789

As a general note, one of the things that I keep relearning in this obsession, is that folks get too wrapped up on the engine and don’t pay enough attention to engine management. All of our shit is old and the replacement cost of some things is crazy. Go price a new engine wiring harness. At any moment that old stuff can start functioning poorly.

#4

I don’t think it is low system voltage that is causing the misfires. But if the alternator was loose and bouncing around, interruptions of it’s ground could have been causing voltage spikes and electrical noise. That could easily upset the DME.

I once had an alternator fail at the beginning of a 30 minute session. The battery lasted long enough to run the session out, but was dead at the end. The car ran fine through the session, but couldn’t be resarted due to the battery being dead afterwards. At least that DME would tolerate a system voltage of less than 12.4v.

#5

[quote=“jlevie” post=57330]I don’t think it is low system voltage that is causing the misfires. But if the alternator was loose and bouncing around, interruptions of it’s ground could have been causing voltage spikes and electrical noise. That could easily upset the DME.

I once had an alternator fail at the beginning of a 30 minute session. The battery lasted long enough to run the session out, but was dead at the end. The car ran fine through the session, but couldn’t be resarted due to the battery being dead afterwards. At least that DME would tolerate a system voltage of less than 12.4v.[/quote]

But consider how surprising it is that the DME would be especially intolerant of voltage problems only at WOT. I could understand intolerance at high rpm because the ignition system would be putting greater demands on system power, but WOT?

It’s hard to understand how alternator grounding problems could cause voltage spikes. Voltage drops, sure, but spikes?

I figure that if the DME can survive the voltage spikes caused by the starter, how intolerant of voltage spikes can it be? Besides, the battery is in parallel and it would significantly mitigate V spikes.

#6

Eric has been experiencing intermittent missing and cutting off at high RPMs in recent events in the 191 - especially as the car gets hotter. The other day we started the car to move it around. 5 minutes later went to start again and we got nothing. Checked everything and could not get a spark. Finally checked the wire coming off the kill switch to the DME, voltage was low - turns out the kill switch is going bad and was causing the cutoff/missing. Looks like we’re replacing kill switches every year.

#7

The is a common enough problem that there has to be a common root cause. I suspect that the contacts in the kill switch that we use for the engine management power aren’t rated for the current lowing through that circuit. Over time that would damage the contacts and result in this problem. A perfect solution would be a better kill switch with higher rated contacts. But using those contacts to control a 60-80a relay would be better than what we have now. That would be easy to implement and I think I’ll do that this coming week.

Along those lines, the fusible link in the engine management power wire is a potential casue of brown outs of the DME. Battery acid can have gotten under the heat shrink and caused corrosion, resulting in a hig resistance path. I had this problem in the new car and when I cut the shrink wrap off I found a mass of white powder and very little of link left. My solution was to splice out the link and install a lug type 60a fuse at the power block between the cable and the block.

Scott,

A bad fusible link or failing kill switch would be consistent with what is/was happening. The DME could have been browning out at high rpm and fixing the alternator grounding could have raised the system voltage just enough to reduce the incidence. If you still have the fusible link, replace it with a fuse and check the kill switch.

#8

The is a common enough problem that there has to be a common root cause. I suspect that the contacts in the kill switch that we use for the engine management power aren’t rated for the current lowing through that circuit. Over time that would damage the contacts and result in this problem. A perfect solution would be a better kill switch with higher rated contacts. But using those contacts to control a 60-80a relay would be better than what we have now. That would be easy to implement and I think I’ll do that this coming week.

Along those lines, the fusible link in the engine management power wire is a potential casue of brown outs of the DME. Battery acid can have gotten under the heat shrink and caused corrosion, resulting in a hig resistance path. I had this problem in the new car and when I cut the shrink wrap off I found a mass of white powder and very little of link left. My solution was to splice out the link and install a lug type 60a fuse at the power block between the cable and the block.

Scott,

A bad fusible link or failing kill switch would be consistent with what is/was happening. The DME could have been browning out at high rpm and fixing the alternator grounding could have raised the system voltage just enough to reduce the incidence. If you still have the fusible link, replace it with a fuse and check the kill switch.[/quote]
I heard a kill switch theory a while back that suggested that the more a person uses it the sooner it becomes a problem. That seemed reasonable to me so I only use mine when I’m working around the starter or alternator.

In order to rule out the kill switch and that fusible link as a possible cause last weekend I bypassed both during one of my tests. I replaced my fusible link a couple years ago.

#9

I’m installing my kill switch this week, and I am going to replace the fuseable link with a fuse. Is 60 amp the best one to use. I know in the KILL SWITCH thread you mentioned a 50 and a 60 amp.

#10

FWIW, I replaced with 30a fuses in both of our cars. Outside of the kill switch issue no problems with the fuses (yet).

#11

Here’s some data for posterity.

Bottom line up front. By cleaning connections and defeating the small wire to the battery I increased DME voltage from 13.61v to 13.78v.

Background. The DME has the ability to adjust to a limited range of supply voltage change. That ability seems to be reduced at WOT and high rpm. Not WOT or high rpm, but WOT AND high rpm. We don’t have numbers for any of that tho. All I can say for sure is that 12.6V seems to be too low. So maybe 12.8 is ok? Maybe 13v is necessary? Likely we’ll never know.

This series of experiments was intended to find out how much voltage could be increased by simply cleaning connections.

Voltage measuring points.

  1. Big lug at kill switch. Goes to battery.

  2. Big lug at kill switch. Goes to big connector at firewall and from there to alternator via the starter.

  3. At firewall connector. Goes thru small fused wire to battery. This is part of the system giving DME it’s own route to batt.

  4. At firewall connector. DME connects here. This is the big enchilada for DME power.

Yes, that’s a little confusing. Think of a 2 pole kill switch, the big pole handling the big cable to the battery and the small pole handling the small wire to the battery. (1) and (2) are either side of the big pole and (3) and (4) are either side of the small pole.

There was a couple other measuring points I used to see if I would find out anything interesting.

All tests were done with the rpms at 1500-2k with no electrical devices running other then dashboard gauges.

Test 1 was the car as it came home from Mid-O.

(1) 13.73v
(2) 13.83v
(3) 13.68v
(4) 13.61v

Conclusions.

  1. The kill switch and the connections associated with it are costing me around 0.1V. (2) is higher than (1) and (3) is higher than (4).

  2. The small wire loses 0.5v. Both (1) and (3) go right to the battery, but the small wire (3) has less voltage.

Test 2 making the connections cherry. I disconnected every connection between the battery’s ground and the engine’s ground, and between bthe battery’s 12V lug and the alternator’s 12V lug. I cleaned every connection with a dremel wire brush and put on conductive grease.

(1) 13.87
(2) 13.87
(3) 13.78
(4) 13.71

Conclusion.
All voltages went up by 0.1V or better. The charging voltage to the battery went up the most (0.14v) and the critical DME voltage (4) went up by 0.1v).

Note how (1) and (2) are the same, indicating no loss across the kill switch, but (3) is more than (4) indicating that there is some resistance in the kill switch’s small pole. This is irksome because it’s directly impacting voltage to the DME.

The small wire to the battery (3) still shows voltage loss vs. the big wire to the battery (1).

Test 3. I added a heavy gauge wire connecting points (1) and (3). This was an attempt to fix the small wire voltage loss.

(1) 13.87
(2) 13.89
(3) 13.87
(4) 13.78

Conclusions. This idea boosted small wire to battery (3) voltage by 0.09v so the idea worked. It doesn’t fix the fact that the kill switch is costing me voltage, but it does increase voltage to the DME (4) from 13.71v to 13.78v. Hard to jump up and down and holler over 0.07v, but at least the various ideas are working as expected and there weren’t any surprises.

A sad final note. Because I’m an asshole, I burnt the shit out of a finger in this test. I tightened up the big lugs on the kill switch without having the battery disconnected so the lugs were “live”. I figured that I’d just be careful about it an there’d be no problem.

I neglected to factor in my wedding ring. Which to my very great surprise, attempted to weld itself between mini socket wrench and sheetmetal. The resulting burns on my ring finger and adjacent fingers were quite “well done” so to speak. Later, getting my titanium ring off my badly burned and swollen finger was fairly challenging. It was a fine learning experience.

#12

If you “defeated the small wire to the battery”, i.e., don’t have it running through kill switch contacts, the engine won’t shut off when you turn off the kill switch. The alternator will keep the engine side of the main power lead hot.

#13

Yes.

When I decided to try the “defeated the small wire” idea, I was thinking that I could come up with a way to do it and yet make the kill switch still “work”. I wargamed a some ideas to do that, but in the end I didn’t come up with an idea that I liked. The various schemes all added complexity to a system not tolerant of such. Maybe I’ll go look at the fusible link that I imagineered a couple yrs ago on the small wire near the battery and see if that’s a source of the resistance.

#14

Yes.

When I decided to try the “defeated the small wire” idea, I was thinking that I could come up with a way to do it and yet make the kill switch still “work”. I wargamed a some ideas to do that, but in the end I didn’t come up with an idea that I liked. The various schemes all added complexity to a system not tolerant of such. Maybe I’ll go look at the fusible link that I imagineered a couple yrs ago on the small wire near the battery and see if that’s a source of the resistance.[/quote]
Did you see my post about the rating on the contacts and a credible solution. When the relay (a Bosch surface mount 75a rated) arrives I’m substituing it for the contacts in the kill switch. I don’t have a problem with my kill switch right now (as proved by data logs), but I figure it is just a matter of time until I do. By dropping the current flow through those contacts in the kill switch to something less that 1a (what the relay coil needs) I figure the switch will never cause a problem.

#15

Scott, Jim…the other alternative is to power the DME (and the fuel pump) directly off the alternator hot wire. Then you don’t really give a damn what the battery does. Chuck

#16

If you do that you have to have something to take away spark when you flip the kill switch. If the DME is tied to the alternator output (or to the engine side of the main power connections, which is the same thing), the engine will continue to run when you flip the kill switch. One could route the coil power through secondary contacts in the kill switch as an alternative to what I’ve previously proposed.

The reason that BMW has the engine management system powered driectly off the battery is to avoid browning out the DME during a hard, cold weather, start due to voltage drop in the primary circuit. That isn’t much of an issue with a race car.

#17

Not sure I understand. Sounds like a 3rd pole on the kill switch would be needed in order to interrupt the wire from the alternator.

Based on excluding the small wire defeat as a workable course of action, I’m left with the 0.1v gain(4) achieved by a day spent cleaning connectors. Not a lot for the effort.

#18

I’ve seen an E30 have the alternator die on track, but not restart after a fuel stop (more 24hrs of lemons). In the course of debugging, the battery was tested to have 8.4V. I was surprised the car ran at all with that low a voltage.

#19

Jim, Scott…a little rewiring. Relay controlled by the switch (key switch) which, in turn would be controlled by the master disconnect. Other side of relay from starter to relay to power for fuel pump and DME. CB

#20

Not sure I understand. Sounds like a 3rd pole on the kill switch would be needed in order to interrupt the wire from the alternator.

Based on excluding the small wire defeat as a workable course of action, I’m left with the 0.1v gain(4) achieved by a day spent cleaning connectors. Not a lot for the effort.[/quote]

I went back and looked at the “small wire defeat” business to try to understand what I was thinking. I was wrong to dismiss the idea, it does work. I didn’t defeat the small kill switch pole, by connecting (1) to (3), I simply provided a bypass for the small wire. The kill switch’s small pole still interrupts power to the DME, and as such I couldn’t get back the 0.1v that is lost across the kill switch’s small pole. So comparing (4) from the start point (13.61v) to the final result (13.78v)I ended up gaining an honest 0.17v. I was kind of hoping to get a 1/2v out of this so the cheering is muted. Lets see what happens when I put in the new regulator.