Charlie944

Hello all! Here is the problem my 84' has developed a series of stumbles at around 3000 rpm + & - 400rpm. I know the stock Bosch L-Jetronic goes lean right at around 3000rpm when under load. This stumbling is more pronounced when accelerating going through the gears than at cruising, but it still is noticeable at cruising at highway speeds. I put spark plugs put in it 10k ago...NGK. The cap and rotor have looked better, but are still functional (no flash over, no cracks, just some corroded terminals). What is a good way to clean the terminals...I was thinking ScotchBright, but I am not sure if it uses Aluminum Oxide for the Abrassive, which would leave a conductive powder in the cap. I am thinking this is fuel related because when my car starts to stumble and I give it some more throttle, it wakes up and goes. Could it be the fuel pressure regulator is going out? The car has 176,689 miles on it and has never had a new reg or injectors.

Any help in diagnosing this problem would be much appreciated!
TIA!!
~ Charlie

Danno

Check your AFM. The wiper-arm may have worn way a portion of the resistor array. Just bend the arm so that it tracks over a new portion of the resistor.

Charlie944

Thank you for the reply Danno! I will have to bust out a couple of my Bosch FI books to get a better depiction.
Appreciate it!

Bri Bro

Look here under AFM for details and pictures.

Charlie944

Thank you for the link beab951, I forgot I had that in my favorites! I will have to get around to putting in a PowerProm in the ole 84' but for now most of my $$ has been going to the Callaway.

What is the best way to get the black cover off the AFM?? I am going to check and see on the wear that the wiper arm has done to the green conductive film of the clock wheel in the AFM (what Danno was talking about) Seems very very likely!

Charlie944

Alright, I opened up the AFM and moved the contact points to a fresh track and while I have better throttle response now I still am plagued with a hesitation around 3000. It is not missing, but feels as if it is throttling back. I was on the highway tonight and running 70, and the hesitation started in and I lost 3 mph. It also is prominent when the engine is just off the first cold field and at that rpm. Only clears up sometime if I go WOT, which I think bumps up the duty cycle via the signal from TPS. I think this is fuel related personally. Any comments please?? I have a bottle of Redline SL-1 in the tank now with Amoco 93oct Gold (clear formulation) that I run religiously. I will also change out the fuel filter when my tranny fluid arrives (do them both while I am back there). Also I am going to check the spark plugs to see about any lean conditions, but it seems to run great at other engine speeds, so I am doubtful of any tell tales signs there. Oh one more thing...my last few tanks of gas I have gotten 50 more miles out of and I am driving the car just the same.

TIA!

Charlotte944

This sounds a lot like what my old '84 was doing. If I had the lights, A/C, or cooling fans running the engine would hesitate between 3500 and 4000 RPM. I could back off the throttle a tad and the engine would rev normally.

I went through plugs, wires, rotor, coil, fuel pump, fuel filter, I tweaked the AFM wipers, injectors, and TPS, but nothing helped.

Finally on a whim I bought an AFM and DME from an '86. When I swapped the AFM and DME the problem went away.

Eyal 951

Hey,
I have a simular problem and am not sure about it. I experience about the same thing, but when I roll on boost. I am 99% sure there is a vacuum leak, cause it idles stable, but a tad high, theres a hiss under the hood, and on the bar gauge, it shows a higher pressure at idle then it used to. When i get on boost, theres a fluttering sound you can hear. I've sked about it before, but nobody has given me an answer. Should i be concentrating on the vacuum lines, or look elsewhere?
~Eyal

Bri Bro

I would be looking for the source of the Hiss.

bloodraven

hey eyal....were is the boost gauge supposed to be at on idle? What about when getting into it a little?

Charlie944

Hey Cliff is the 86' DME a direct plug-in replacement for the 84' DME?

Eyal 951

Well, I don't know where it is supposed to be, I can only assume its supposed to be where it used to be, which was right below the .4 mark. I messed around under there today, tightening what I could, looking around, messed with the lindsey boost enhancer (don't know what I'm doing with that) and now the car idles high at a tad less then 1100 rpm, but seems to boost stronger. The boost now comes on past 3000 rpm, spiking at maybe 3800... which is way higher then it did before i messed with the boost enhancer, so I think I'll turn it the other way
~Eyal

Charlotte944

Yeah, but you have to swap both the DME and AFM. The early DME sends an unregulated 8 volt refereance voatage to the AFM, and the later DME sends regulated 5 volts.

If you just swap the DME the engine load signal from the AFM will be WAY off.

An additional upside is you can chip the later DME.

Charlie944

Alright I replaced the fuel filter yesterday morning and the car still stumbles around 3000! Thing is the car does NOT do this EVERY time which is making it hard to diagnose. When it starts I push the throttle down gradually and it usually clears up at 3/4 throttle, sometimes not though which I then mash the throttle and it clears up and goes! The WOT clears it up every time when the TPS sends signal to for the injectors to step up the fuel. I am still getting 30-50 miles more (depending on how long I drive on the reserve) per tanks of fuel.

Could a faulty fuel press. regulator cause this?

I did replace the vacuum lines that stem off the Throttle Body with Purosil. I also checked around the engine leaks while running at idle with a propane gas.
I want to get this stumbling licked before I go to Hershey for that would get quite aggravating for 1500 miles round trip.

TIA!

Charlie944

Propane gas test for leaks was negative...no leaks that I could detect