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Light Load Stumble/Miss


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1977 L24E with Bosch L-Jetronic

 

Symptoms:

 

Fully warmed up (~10 minutes of driving), light load (up to ~1/2 throttle) stumble/miss.

Cold or cool, no stumble/miss.

More then 1/2 throttle, no stumble miss.

Full throttle no stumble/miss.

Idle no stumble/miss.

 

Checked/Fixed:

 

Static compresison good in all cylinders and all within 5% of each other.

New generic fuel injectors.

New engine temp sensor.

No vacume leak found.

New fuel filter.

 

Unknown:

 

Fuel pump.

Fuel pressure.

 

Diagnosis:

 

Have been told that the stumble is a normal emisisons lean trait. I don't want to believe that. Anyone have any good ideas?

 

- John

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Throttle valve switch maybe? (thing that looks like a TPS on the side of the throttle body) Could have a bad contact, or be out of adjustment. Or... You say 'new engine temp sensor' but there are actually two. One for the guage, and another that is called the 'thermotime' switch (I think) that tells the computer the engine temp. Mayby it's starting to fail/mis-read... hmm.. you sure the vac-advance is good on the dizzy? The line might be fine, but the internal diafriagm might be torn/corroded... I dono.. I guess it also might be an AFM problem.. but I would think it would do the stumble/sputer thing a lot more than you say...

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John, I am not sure I can offer a lot of help here, but I seem to have a similar problem plauging my car. Basically everything in my car is new or has been changed with the miss still existing... the only things I really suspect are:

 

1) TPS

2) FPR

 

I have a fuel pressure gauge, and from what I can tell the miss does not correspond to fuel pressure changes of any type. I am sure that it is not a lean/rich miss either as I've played out that scenario to death with the SDS mixture meter.

 

Check the TPS setup, swap it if you have a known good spare, and then I'd move on to fuel pressure/FPR if that doesn't solve it.

 

Not much concrete to go on I know... I'm interested in hearing your results though! (I should do this for myself too, but don't have a spare TPS at present and refuse to buy a non-returnable electical part on a whim, as I've never been able to have the TPS replicate anything like this while not driving)

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Early TPS is basically a double contact switch that give the EFI a signal at idle and on a different pin a signal a WOT. It can cause problems if either side of switch is bad but you can disconnet it. It may not idle perfectly or give proper enrichment a WOT (wide open throttle) but if your symptom goes away you have found your problem. The plastic housing cap can be easily removed and the contact point rebent to give proper function. The adjustment is continuity between center contact and one of the side contacts with engine off/throttle closed and this continuity shold be lost as soon as you crack throttle open. After adjustment of TPS recheck this a few times to verify TPS internals have "taken" the new set. TPS is adjusted by loosening mount screws and rotating housing. Sometimes you have to readjust a few times before it works properly. After adjustment check that you get continuity between center pin and the other side pin at approx 80 percent of WOT.

This check/adjustment cost nothing and is fairly quick.

Fuel pressure should also be checked to rule it out as a contributer to you problem but the fact that the car runs ok cold/cool tends to elimanate fuel pressure as a factor. Is this vehical "California Emissions". 48 state vehicles did not have "lean cycle mapping" back in late 70s but Cal vehicles may have.

This era L-series distributor has a habit of the breaker plate bearing retainer(plastic) failing and allowing the breaker plate to sag on one side causing erratic operation of vac advance and contact between the mag pickup tips and the senor. This is usally easy to spot as one or two small ball bearings will be laying on the breaker plate fixed plate. 80 percent of dist I inspect have this problem. Fixing this usually vastly improves drivability. Fix is to replace breaker plate (60-75$ part at Nissan dealer)

Check AFM flap to see if it binding in housing (try to check as installed. I once had one that would check fine on bench but had mild interferance problem when installed). Any contact between flap and housing during operation is bad.

At last resort adjusting AFM spring can change mixture. Loosening spring a couple of notches may richen your mixture enough to solve lean issues.

Might also overcome factory "lean mapping"

Might also violate emissions laws- Never do this for profit, its a felony. Doing it to your own vehicle is a misdemeanor (at least in Tex)

Hope this helps out

Rick

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Not to hijack this thread, but I have essentially the same problem on a ’92 BMW 325is that I just bought; it has a later-generation Bosch engine management system.

 

Does not stumble/miss when cold. Happens when the car is warm, below 3000 rpm, also at idle. No problem above 3000 rpm. Occasional backfire through exhaust, accompanied by sulfur-like smell. Idle when warm is unstable (hunts between 600 and 1100 rpm).

 

These cars have an individual ignition coil per cylinder (no distributor). Neither timing nor mixture are adjustable.

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Thanks for all the responses. I've checked and adjusted the TPS, but I didn't check it the way Rick said, so I'll try that first. The air flap seems fine. I can get part of my hand and a probe in there and I feel a pretty steady, smooth movement as it goes through its range of motion - but that's with the car cold.

 

The temp sensor I replaced is the one that goes to the computer, which is not the thermotime switch. I might just go back and replace the thermotime switch but its $60, so I was trying other, cheaper solutions.

 

I had not thought of the problem as being ignition, but it might be a vacume advance thing as mentioned. I'll pull the distributor and inspect it.

 

The car is CA emisisons car with a catalytic converter and does have the altitude switch. I have a new cat in the car.

 

Hopefully I get some time this week to tinker with it some more. Thanks again for the advice.

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