stumble under acceleration

surfdog4 · 06-17-2008, 07:35 AM

Well i have been making good progress on the electricals after the cluster rebuild, new alternator, etc etc. (all idiot lights are now extinguished!) So of course now something new has come up

Every so often i have been taking the car out for a ride (to remind myself why i am restoring this thing)..up until recently things have been mostly uneventful..now seems like i have acquired some new engine issues.

When i get up to highway speeds and hit the accelerator, the car stumbles and stutters like it doesn't want to go. Another interesting thing is before the idle seemed to have been stuck at about 1000 rpm..cold or hot..now while sitting at a light after warm up it will drop to 400/500, like it's going to cut off.

Prior to all this I had replaced all the vacuum lines I could see under the air assembly (still have some in the front to go). The timing belt was loose so I retensioned it..and it has not been driven 50 miles since i retensioned it

I was not getting any temp reading at one point but cleaned up the contacts on the sensor unit and now get readings on the gauge (not quite up to halfway).

part of me is wondering if there is a computer issue that it was in some kind of default mode when it was not getting info from the temp sensor.

I'm making the list of things to do

-I have a new fuel filter but have not installed it. (I had not put much gas in it and was running a high concentration of Techron until recently) Also, the last gas i got had 10% ethanol, not sure if others are having problems with that. maybe i stirred up some gunk in the gas tank?

- I really don't know the status of the plugs but since it seemed to be running fine before i had moved that down the list..

- from the parts box i got, some PO had replaced the rotor and cap, again not sure when but it was running OK before

- I'm going to pull the air cleaner and make sure the vacuum lines i replaced are still connected

I'm open to suggestions of where to start..thanks

Also, on the temp gauge, can someone tell me what "normal" should be..right now my gauge when warmed up is not making it to midpoint on the dial..so the sensor is working but i'm not sure if it is working correctly..the "ohms" at the sensor seem ok per specs but the range is very wide

surfdog4 · 06-19-2008, 05:22 AM

c'mon guys...no takers on this?

erwalker · 06-19-2008, 06:57 AM

I have had a similar problem (82 MY) and had a few replies to my post but have not been able to nail down the problem yet. Mine is fine when first started but once it has warmed up when I hit 3000 rpm the engine starts to misfire to the point where she will not accelerate any further. I've been advised to check wires (done with no improvement), coil, cap, rotor (done) plugs (done), timing (done), vacuum lines, electrical grounds (done), electrical connections to ignition unit (done), the "green wire" to the distributor, and I am sure there are another 200 things to look at. Since my original post i have looked at the timing belt and found it to be one tooth out on the left bank of cylinders so that has to be corrected. That is also the side that drives the distributor so not only is my valve timing out but the dist is not out of whack as well. I'll be watching your post to see what you come up with. If I find my problem, I'll update my post as well. Good luck!

Danglerb · 06-19-2008, 07:01 AM

On my car that has the nice radiator and factory fan the temp goes a hair over the lower white line and stays there. Generally the gauges are not absolutely accurate, just an indicator of normal or not behavior. If it goes where it usually does, don't worry.

I'd start with air leaks.

Does the distributor have a mechanical advance mechanism? Working vacuum advance?

surfdog4 · 06-19-2008, 11:48 AM

thanks Danglerb..my problem on the temp gauge was that I had no reference..the gauge did not work at all when i bought the car so I didn't know what "normal" was...now it is at least consistent

Danglerb · 06-19-2008, 07:32 PM

Erwalker, check your timing with a timing light.

erwalker · 06-20-2008, 06:42 AM

I've checked the timing with a timing light and it is set according to spec at the designated rpm. However, with the timing belt on the 5/8 bank gear out by one tooth, the timing adjustment may be out, despite correct readings, as that is the bank that drives the distributor. Also, with the valves being out of time slightly, can't really go any further with a diagnosis until that issue is corrected. Once the "one tooth out" is fixed, I can look at other possibilites if the misfire is still exists. I suspect the misfire will still be there, but I'm sure the out-of-synch valves will not be doing be doing me any favours as far as power is concerned.

surfdog4 · 06-20-2008, 08:40 AM

so there's no chance your problem (and maybe mine) could be the fuel filter since it happens only at higher rpms?

erwalker · 06-20-2008, 08:59 AM

Thought of fuel restriction problems as well. In my case I replaced both the fuel pump (original was leaking at the top seam) and the fuel filter (precautionary) so those two items have been ruled out although there may be other restrictions in my fuel system that I have not yet found. A fuel pressure test should expose any problems although I have not yet done that. If your filter hasn't been changed in a while, that might be a good start.

Danglerb · 06-20-2008, 10:09 AM

Is the vacuum advance mechanism working (erwalker)?

Fuel restriction will go lean under load and eventually cut out, but I don't think its going to "feel" rpm dependant.

Problems like these its so nice to have a second car available to swap parts with.

erwalker · 06-20-2008, 10:22 AM

NOW I understand why so many people have two or more 928's - they're easier to fix in bulk! Interesting point about fuel restrictions - I've always assumed that as long as the system was able to supply fuel as the system demanded it, the engine would run fine but at the point where the system cannot supply enough fuel into the injectors due to a restriction, the engine would start to hesitate and/or miss.
As far as the vacuum advance, both the vacuum connections to the distributor hold vacuum and do not loose it. But I have not checked the vacuum lines with a vacuum guage yet so that is worth a try. They appear to be in good condition but there could also be internal blockages, etc. I'll have a look at that this weekend. Valves out-of-synch can also affect vacuum so another compelling reason to get that issue cleared up. Thanks for the ideas!

Danglerb · 06-20-2008, 12:29 PM

Greg opened up a 30 year old distributor a few months back and the vacuum advance mechanism had never been touched since the factory. Rusted tight for a good 10 years or more.

What I was thinking with the timing light is if it shows a good spark and reasonable timing during the miss then its likely a fuel and not a spark issue and vice versa.

DanielDudley · 06-21-2008, 10:48 AM

Did you disturb the plug wires ? Can you verify fuel pressure ? Pressure regulator could be shot, as could the green wire running to the distributor.

I personally had similar symptoms to yours when my timing belt slipped. Verify cam timing, then verify spark and electrical components, then verify fuel pressure . Check for a good air fuel meter connection [AFM], and make sure the temperature sensor connections aren't corroded. Recheck all the vacuume lines you replaced, and all electrical connections near or ajacent to these lines. Blocked fuel filter would cause loss of power, but is unlikely to effect idle. Should run OK with a bad O2 sensor, but not great.

Many people ignore routine service items on 928s because they cost money, and the car continues to run. The infamous green wire cn look good fron the outside, but perish from internal breakdown. In general, fuel problems are like a cough, cough, cough, spark pproblems are like a stutter, stutter, as in one second fine, next instant nothing, back and forth. Plug wires or connections that seem fine can suddenly fail when disturbed.

Many times in the past I would perform a routine full tune up before diagnosing a ''Fuel related'' problem and have the problem disapear. Just because you have an old cap in a box doesn't really say how fresh your plugs are, or anything else. Could be an heirloom cap passed on from owner to owner.

I recently saw my old 633CSI for sale by the road. I sold it on the proviso that it needed a tune up, but ran fine, for 1600.00. It was for sale over a year later with a blown clutch for 4,000.00 ''26,000.00 invested''. Yeah, by the two owners previous to me. I did struts, valve adjust and muffler weld. Tune up was at least five years old, my estimate. Et TU ?

surfdog4 · 06-21-2008, 06:23 PM

what's interesting is i didnt have this problem until i cleaned the corrision from the temperature sensor (!)

surfdog4 · 07-07-2008, 04:52 AM

OK an update..yesterday took the car out and it didn't want to idle at all at first..kept cutting off.

Once i got it driving, it basically stumbles/misses whenever you put load on it..if you get the engine RPM's up, it runs fine, but if you try to accelerate, it hesitates and stumbles.

Once i got it in the driveway, there was some smoke/steam coming out of the air intake..but it stopped before i could get the air cleaner out. No obvious vacuum lines disconnected, although the breather lines connected to the air cleaner were off.

It's wierd that this just started

Danglerb · 07-07-2008, 08:48 AM

My vote is that something broke when you reached in to fix something else.

With a big V8 motor it can feel like its "running strong and pulls hard" and actually be running terrible down substantially in power with many things screwed up.