Forgive me for starting a new thread that is really just a continuation of a thread I started last year for this same issue. I've tried a couple of new things since then with no progress and figured I'd start a new thread to consolidate everything I've tried so far on one post. (I also put a link at the end of that thread to this one).

Symptoms:
Car will intermittently go into a very lean state causing the power to fluctuate, RPMs to fluctuate at idle and often stall if not opening the throttle a fair amount. Usually happens when engine is warm, but will go away mysteriously on it's own- usually after over enriching the air/fuel mixture and running in that way for several minutes.

I’ve determined it’s going lean because when this happens unplugging the TS1 / Air temp sensor and the surging / staling will cease. (Unplugging the TS1 according to pbanders site enriches the fuel mixture)
Or I can dial the ECU or MPS for a richer mixture which will also solve the issue to certain degrees, but when the issue mysteriously goes away the fuel mixture is too rich.

Symptoms are similar to when an MPS diaphragm fails. In fact, the car was running fine up until about 3 years ago when the MPS failed. I bought a rebuilt / recalibrated one from Jeff B. The car wouldn’t start with the rebuilt MPS because it was too lean (or rich, I can’t remember). I adjusted it, it started, I adjusted the AFR using a wide band sensor and ever since it’s had this intermittent issue.

Things I’ve done to try and solve the issue:
-Ensured correct matching ECU, MPS and CHT sensor.
-Have tried a correct # spare ECU
-New CHT
-Verified CHT is in spec both when engine is cold and warm. (I checked the CHT when the lean condition happened and it measured 78 ohms).
-Installed 123 distributor (issue also happens on stock distributor). I’m running retard on distributor, but have tried without.
-Installed new coil with the 123 distributor. Issue happens with old (stock) coil, too.
-Replaced all vacuum lines and verified they are hooked up correctly.
-Cleaned and flow tested injectors
-New injector seals
-Installed fuel pressure gauge in engine compartment- ~28psi. Reading is constant even when surging/stalling happens.
-New intake runner seals
-New intake runner gaskets
-New throttle body gasket
-Swapped throttle body with NOS model.
-Verified no vacuum leaks with smoke test.
-Verified AAR is opening/closing correctly.
-Verified the decel valve is opening/closing correctly.
-New injection harness
-New ignition harness
-Swapped relay board and verified continuity on all connections.
-Tried a spare rebuilt MPS that hold vacuum
-Manifold vacuum is constant at 15-17 HG
-Smoke test didn't reveal any leaks.
-Verified decal value is functioning correctly
-Monitoring the AFR using wide band sensor.
-Calibrated throttle position sensor. Tracks look good. (Unplugging the TPS has no effect when issue is happening)
-Verified grounds from the FI harness, under the relay board and ground strap on tranny is good.
-New odyssey battery (verified voltage is good)
-Verified timing and valves are adjusted to spec.

Misc info:
-Setting the idle above 13:1 at idle or thereabouts puts the idle speed at 1100rpm, and it will hunt unless you richen up the mixture, either with the ECU knob.
-Throttle body idle screw is all the way in to keep the idle at ~ 1000rpm. Verified that air gets past the idle screw from above the butterfly. Apparently not an uncommon issue according to others. Others report having a hard time getting the idle below 1000 with the 123 ignition.
-Compression is low on cylinders 1 & 2: 110, On cyl 3 & 4: 131
-Open AAR valve doesn't increase engine RPM when engine is cold. Verified AAR is opening. However, plugging the hose that runs from the AAR to the airbox will stall the engine until it's warmed up. (AAR closes as it should then)

Fuel Pressure?

Edit: Nevermind ha

Pretty comprehensive list.

I know you said you smoke tested it - How sure are you? Have you been able to smoke test it when the issue is ocurring?
Didn't Zach just find a loose throttle shaft causing similar intermittent issues?

When you say it goes very lean, what is the AFR when that is occuring? Does that give any insight into how much excess air is entering somwhere?

Also, since you say disconnecting the ambient temp sensor TS1 helps, are you monitoring the resistance of that TS1 temp sensor when it is occuring? Could that sensor itself be going intermittent?

Thanks for the response. I'm not sure of anything anymore.
I was thinking about smoke testing it again. That's probably worth doing again, although wouldn't a vacuum leak with DJet result in higher rpms at idle and not a surging/stalling condition?



When it goes lean, it fluctuates from lean to rich pretty quickly- the idle hunts from ~1100 to almost 0 (lean), and then will stall after several oscillations if I don't richen the mixture, similar to what happens if you turned the ECU knob all the ways counter clockwise. AFR gauge tries to keep up but goes up and down with the idle surge and goes past 16:1 when the RPM drop, and after several cycles will stall. I can keep it running if I keep the throttle open, but power is way down and I can still feel it oscillating.
Then there are times where I'll be driving along fine at low or high speeds with no hint that anything is wrong-AFR even looks good, but if I put it into neutral the car will stall. This is also intermittent, but when it does happen it's hard to start without opening the throttle all of the way, and then the idle oscillation behavior happens.



I don't think so, because when I unplug it and then adjust the AFR accordingly, the lean issue/hunting idle/low power/stalling still happens.

I was really hoping it was just a simple fuel delivery issue, but the gauge reads a constant 28 psi at all times.
The characteristics really seem like a bad MPS (I have 2 that hold vacuum that I've already tried swapping) or a bad CHT. My CHT reading was 70 ohms last time this happened. Is that too low? Pbanders site says it should be less than 100 ohms with a hot engine but doesn't cite specific numbers. I've tried swapping the CHT out too though. :/

i'm unsure what type of EFI you have.

but i had a similar problem with a 74 1.8 L jet earlier this year.
guys here helped to point me in right direction.

with mine it was fuel pressure.
and it was heat related.
only happened when ambient temp here in aus got up into the mid to high 30s (centigrade) and beyond.

in the end i worked it out with clues from other members.

i had changed orig plastic fuel lines to steel.
i had replaced original fuel pump with newer in line unit.

all these added up.
i was boiling fuel in the steel lines after exit from firewall due to radiant heat from fan casting or just general heat build up under car while sitting at idle at the lights.
i was constricting fuel in the tighter bends in flexible fuel line from the fuel pump up to the engine bay due to fuel pump being longer and making that flexible bend tighter.
or all of the above together.

its cooler now in aus and the problem has gone away without me getting around to changing any of the above.

admittedly there was one other symptom that arose at same time which confused matters, namely a leaking throttle body gasket, but fixing that made the problem less severe, but did not make it go away a the time.

but i've got some insulation shielding for the metal fuel lines.
i've rebuilt my original fuel pump.
and i'm ready to change it all over.
i expect to ease both problems listed above.

but it would not idle steadily and hunted and it would cut out.
and i worked out it was fuel pressure problem that came in under hot conditions as heated air built up under the engine bay.
at all other times i was getting perfect fuel pressure.
i could hear the pump making beserk noises when it was stressing to get fuel and the problem was happening.

hope this helps. may not. the guys here really helped - they kept steering me to fuel pressure and ......but yours may not be that

This seems like a really important clue. So if I understand your description correctly, a rapid change in load (i.e manifold pressure) can't be handled properly.

Maybe lends some concern for a MPS like others reported that had hairline cracking of the diaphram yet it still holds vacuum. Would be more concerning but I think you seemed to have that covered via rebuilt unit and same behavior with a separate MPS.

On the other side of that would be the TPS sensor properly reading a return to closed throttle postions (assumed in neutral) and the fact that it doesn't want to restart with a closed throttle. But you seemed to have that one covered too with trace check and new harness.

When things are intermittent I tend to lean toward electrical issues. Yes, intermittent issues occurs with mechanical devices but less common.

Any chance that you can drive and leave the ECU connector accessible and unplug and replug it when the issue is occuring and then see if it clears up? Probably dependent on how often this is happening and I hate to have you driving around with it unsecured for long.

Not sure what CHT p/n you're using but as a generalization I don't think 70 ohms is too low. Here is some data that we were gathering around 73' 017 CHT's back at start of the year. I measured a 017 down to 30 ohms at 350F.

http://www.914world.com/bbs2/index.php?showtopic=351065

Yeah, agree that minor leak woud be higher rpms (i.e. not able to pull higher manifold vacuum at idle). I was wondering if it could be some sort of much larger leak that was getting in and not being measured by MPS and leaning things out.

Thinking of large leak paths. It almost sounds like the decel valve might be oscillating. When you say decel valve is OK - is that measured against vacuum levels and at what vacuum it opens, or was that simply that it opens and closes with vacuum? Set point needs to be above idle condition and only opening the valve when engine is over-running with high vacuum that you won't see at idle. On over-run, modern cars will do a complete (or nearly complete) fuel shutoff for fuel economy. Not uncommon to see AFR of 17:1 or even leaner during that decel fuel shut-off conditon as fuel is feathered in & out at the DFSO transitions. Wondering if your 16:1 condition and hunting is indicating a major air bypass that is oscillating due to improper set point?

If it isn't going lean due to air bypass then the only other option is that fuel is being reduced to cause the lean condition like 16:1. Since you're saying fuel pressure doesn't vary, it seems that it is more likely to be going lean due to the airflow path. Maybe a quick check would be to rig an LED to the injectors so that you can verify that when it is oscillating that you don't have injector pulses cuting out. Ideally, I'd use an oscilloscope but betting you don't have one and/or that you won't have it when the intermittent condition occurs on the road!

I've tried two different decel valves, adjusted them both to vacuum numbers I found on other threads (I can't remember the number off-hand at the moment) and verified with a vacuum gauge. I haven't tried completely removing the decel valve from the system though.


Checking the injector pulses is also a good idea. I can do that with a test light, the hard part will be keeping the engine running when its happening while trying to check.

I really appreciate the help and ideas, thanks!

I like a good mystery.

You've really got this one covered. I'm out of ideas for now.

Update:

I may have tracked down my car's intermittent surging idle and stalling to a bad ECU- or two of them, to be exact.

Last year when troubleshooting I acquired what I assumed was a good ECU. The stalling continued with the 'new' ECU, so I chalked it up to another FI component as the source of the problem and reinstalled ECU #1.
Last weekend for no good reason I installed ECU #2 again. When the car stalled after 20 minutes of driving and wouldn't start, I unplugged the TPS and it started right up. Re-connecting the TPS immediately caused the car to stall. So I left the TPS unplugged and drove around without any issues for an hour. Hmmmm...

Test results with each ECU:
ECU#1: Intermittent fluctuating idle due to intermittent lean condition, low power on partial load, often stalling when throttle is closed. Unplugging the TPS doesn't change anything.
ECU #2: Idle is good, but will stall after warmed up. Unplugging TPS allows car to run fine.

Since I have a swanky new FI harness courtesy of Jeff Bowlsby, and swapping out the TPS didn't change any of the above, I'm confident that both of the ECUs I have are bad. I know the mantra is that they rarely fail, but if in addition to the two ECUs mentioned above, if I also count the NOS ECU I bought off of eBay last month that caused the car to run super rich and flood the engine (I had to clean the plugs and drain the oil), and the one that failed before I took ownership of the car (as evident by the words 'known good' scrawled on ECU#1 with a sharpie) my car has experienced 4 bad ECUs.

It's clear that ECUs do fail, and at least in my case, fail fairly often. It would probably help extend their life if they were wrapped in some sort of heat resistant sleeve.

So now with fingers crossed and high hopes, I'm looking for an x-043 ECU. Car is a '75, but I would prefer not having an ECU with the overrun circuit that the later ones had.

Hey Robert --

Cautious (again). I hope you've finally ferreted out this annoying issue.

I do have an 043 ECU (actually 2 I think) if you need a long term loaner

Double double…

Hey Bob, if you decide to toss your bad ECU, please send it to me. I will test it to verify that it’s bad with my test equipment and possibly road test it on my ‘74 2.0L car. No charge. I’ve only seen a couple bad ones.

Yes, they do go bad. The one in my car when I purchased it failed after a year. Sent it off for repair and after 6 months it failed again. Spent all kinds of time and money trying to track down problem because I was sure it couldn't be the ECU! Bought a replacement off of Ebay and it's been working for last 6 months (knock on wood). Guess you and I are in a pretty exclusive club!

I think people probably throw them back in circulation, too.

Happy to report that a replacement ECU seems to have done it. Drove around in hot weather (high 80s) for an hour without any issues.
A big thank you to all who suggested things to try.

Special thanks to Bruce (bdstone914), Rob (BeatNavy), Mark (mepstein) and Jeff (JefBowlsby) for the parts. Sorry if I left anyone out. And thanks to everyone who ever contributed to the content on this site. All those DJet threads I’ve read and re-read really came in handy.

So there- now I’ve probably just jinxed myself.

and a

Good stuff, Robert. Hope it's smooth running from hereon!

And, just for the record, when I first got my teener I'm pretty sure my 1.7L ECU was bad. Chased a non-working fuel pump for weeks before finally swapping out ECU's. And it worked. Perhaps not often, but they can and do go bad. And just like everyone here, each new day they are older and have more miles on 'em!