safulop

OK, past few days I have had considerable hesitation, but only at wide-open throttle. Get's worse as rpm climbs, and sometimes the engine is missing so bad or something that it will not get past 4000 rpm until I lay off the gas.

Possible water contamination of the fuel; I have tried to address that and it has maybe improved today but I think it is likely something more serious with the fuel lines.

Is this filter, or pump? I'm starting with filter and go from there.

Speedtoys

Id look at plug leads at the coils too...or if moisture is geting in the caps.

safulop

I just got brand new plugs, plug wires etc. in August, so unlikely to be ignition related. But maybe the coils did not get proper attention, I will keep that in mind.

Speedtoys

Ya..fuel delivery or spark on load....funny how spark issues get worse as RPM goes up..when thats the issue. LIke..after an engine bath, etc.

James Bailey

Will it rev slowly to over 4,000 RPM ?? not at wide open throttle ? Because if it will pull to red line it is NOT a fueling issue..

Koenig-Specials 928

When mine acted up like that it was the coils. I replaced them with MSDs and never looked back. 3 years ago.

safulop

From what I have seen so far it is free-revving in neutral, it will rev up to wherever with no problems, and very quickly. There is no stumble in neutral. It will also go up slowly to redline smoothly while driving as desired, only not at large or wide open throttle input. I have read that hesitation and misfiring under load comes from fuel line supply problems. Merely revving the engine does not necessarily require max fuel demand, but WOT under load does.

Am I wrong on this?

safulop

Thanks for the tips, I will definitely put this on the list of possibilities.

dr bob

Sean--

Could be pump(s), or the filter, or the regulator. Ore electrical/ignition. I know we replaced the filter on your car in the year before you bought it. Not that replacement then would rule it out from being a problem now...

Focusing on thee fuel delivery system: Check (or have your shop check) the fuel pressure and delivery volume. Procedure includes delivery with engine off, pump jumpered. With no vacuum applied to the regulator (engine is off), the pump sees the maximum available discharge pressure. Fit the jumper with a toggle switch or (preferrably) a button in series, leads long enough to reach out to where you can use the button to activate the pump. A "remote starter button" is perfect for this duty, just add male spade connections to the wiries so you can plug them into the CE panel in place of the FP relay socket.

You'll want a container that holds at least half a gallon, and a way to measure the fuel in the container. A used but cleaned 5-Qt Mobil-1 jug does the duty for me handily, since it has a semi-transparent window to verify level inside. It can be tough seeing fuel through the window, so a dipstick of some sort is often needed. Or just measure the fuel quantity using your handy measuring cup before putting it back in the tank.

The factory rating for fuel delivery is minimum 1350cc every thirty seconds. Push the button to run the pump with the return line from the engine sitting in the container. Using the bigger container allows you to run the pump for 60 seconds, and look for at least 2700cc (2.7l) in that time. Verify that the system pressure is at 2.5bar +/- 0.2bar during the test, usinf a pressure gauge connected to the test port at the front of the right (passenger side on US cars) fuel rail. This testing alone won't tell you if the problem is specifically the pump(s?) or the filter, but would help determine if the problem is in the fuel supply/delivery system.

Flow specs are listed in the WSM at 24-121. Section 20-5 has a diagram for testing that's listed as "Beginning with 1984 Models" but the hose configuration shown is less than stellar. It's sometimes easier to connect to the return line at the fuel cooler, rather than removig the whole air cleaner assembly for access to the hard line connection.


For those playing along at home, Sean describes classic fuel delivery symptoms (flow, pressure). Of course, there are other things along the way that can cause these symptoms, including ignition and associated stuff.

There's also the Stupid Stuff list, which for these symptoms includes the fuel pump relay and FI relay, and the dedicated connections for those systems at the battery positive terminal. If the Critical Systems relays haven't been replaced yet, I'd start with those just because they are relatively cheap and easy to rule out. Fuel pump, LH and Injection, and EZK relays are the ones that will strand you. Clean the positive battery connection, with full attention to the smaller wires that connect there. Those smaller wires feed the systems described above, plus the cooling fans on the S4+ cars. Any of those relays or the non-fan connections can cause the symptoms described.

safulop

Are those "critical systems" all the old #53 relays? I might still have an original stuck in there. . . but I have a spare ready.

My fuel filter is actually new from last spring, but you never know, there could have been some contamination or debris that got into it. The problem sprang up fairly suddenly on Friday, we went from flat out awesome one day to the little engine that could the next. That's what made me think maybe it was a sudden clog in the line, or a sudden electrical issue. But because it starts and idles OK I thought it would be a mechanical fault.

The positive battery connection, you mean the one actually at the battery under the floor? Stupid questions I know. . .

James Bailey

If while driving it will pull to red line at moderate throttle it is probably not fuel delivery ....that leaves electrical.

dr bob

Relays: Consider "storing" all your new spares in the CE panel.

Battery connections right at the battery positive post and clamp, under the spare wheel in the back. Lift the ground strap at the rear bulkhead before doing anything in there.


Can you identify anything that you did somewhat prior to noticing the symptoms? Got a tank of gas at the Terrible Herbst station, now it does this? Or even at your normal fuel station. Otherwise-- "The car seemed to run fine until... it didn't!" Something changed, we just need to figure out what.

My car has always been great, until one day I decided to drive from Glendale to Anaheim, to meet Rob and GB for lunch one day. Before the end of the [long downhill] driveway, it was running really poorly and wouldn't restart. It had been sitting with the battery disconnected for a while, so I started off by disconnect/reconnect to make sure I didn't miss something. After dragging the car back up the [long, uphill] driveway, the fuel pump relay (original at 100k) turned out to be the problem. Not at all associated with the battery disconnect/reconnect, storage, restart, whatever. It just "decided". Jim Bailey's sage advice for years has been "relay-relay-relay" when erratic stuff shows up, and I can say that this advice has been spot-on more tomes than not. Add in corroding connections and grounds, and a massive percentage of common 928 age-related maladies can be avoided. "Store" your "spare" 53b relays in the critical slots for injection, fuel pump and ignition. Decide if you want to keep the tired/questionable ones as dead weight, or just buy a couple new extras for the time your horn doesn't work.

AirtekHVAC

Ya know, you are describing what my 82 is doing. Runs perfectly at idle, in neutral I can go full throttle. Under hard load/wot, misses, stumbles, and lose all power. If I feather the throttle, I can get up to a good speed...(had as high as 80mph)....new fuel filter.

It will not run at all, idle or otherwise with the vacuum advance connected. Above with vacuum advance plugged. I thought it was cam timing......

Following with much interest......

James Bailey

1982 and S-4 have almost NOTHING in common....might as well be a Chevy and a Ford...

V2Rocket

Take a look at the DME temperature sensor.

I had similar-sounding issues on my 944 years ago, would just fall flat at WOT but would be ok accelerating at light throttle. Faulty DME temp can make for lean mixture on Motronic/Jetronic. Running lean can feel like you have half an engine, or less, very down on power.