930 Stumbling When Warm

[Adam144]

Still having issues with the 930 race car. The car is basically stock in the engine except for MSD 6AL, Taylor spiral wires, blaster coil and an inter cooler.

What's happening is the car is stumbling and hesitating after a few laps. It will run the way it should with good AFR readings on the gauge and the in an instant it loses power and stumbles. It becomes un-driveable and doesn't stop acting this way til it cools down.

We have installed new plugs, new wires, checked all hoses (most are brand new as well) and are running out of ideas. Compression is good. Fuel pumps are working. Could it be the warm up regulator???

[Adam144]

Any input as to how the warm up regulator could affect this or produce these symptoms? I can't think of any other component that would change the way the car acts when it gets hot.

[Ronnie's.930]

A vacuum leak will do this as well. So you have verified that all intercooling and intake components are tight and leak free? Recently, another guy on here had a similar problem and he ended up finding a hole/split on the bottom of the large rubber "elbow" connected to the output of the metering plate assembly . . .

[wjfk32]

^^^^^^^^

Everything he said...tightened up hoses and bolting down intercooler better and airbox...
Checked connections to plugs and cap and rotor..etc..etc..

Changed plugs--went with W-24 denso-copper-right out the box-28-32 gapped..

All good. I use mmo/gas on every fill up.

Walt

[RarlyL8]

What did your AFRs do when the engine starts running badly?

Heat can cause aging electrical components such as relays to momentarily fail. If one of your fuel pump relays is affected that would do it.
You might check all the aging electrical components as racing is a tough environment.

[mark houghton]

Just to add to Brian's comments, if you do indeed have poor electrical connections - particularly to the WUR, that could cause you to develop a rich condition when the car is warm (no power to the WUR would cause it to revert back to cold start enrichment settings). The same could happen if you lost the rear fuel pump relay, since the power from that is what feeds the WUR.

[Adam144]

Thank you very much for the replies I appreciate it:

Originally Posted by Ronnie's.930

A vacuum leak will do this as well. So you have verified that all intercooling and intake components are tight and leak free? Recently, another guy on here had a similar problem and he ended up finding a hole/split on the bottom of the large rubber "elbow" connected to the output of the metering plate assembly . . .

We installed brand new multi-layer silicone hoses and re-inspected each and every one of them. Been all through every single line in the engine bay and found them all to be in very good/brand new condition. Also sprayed all the joints to see if the idle speed picked up at all and found no issues.

Originally Posted by RarlyL8

What did your AFRs do when the engine starts running badly?

Heat can cause aging electrical components such as relays to momentarily fail. If one of your fuel pump relays is affected that would do it.
You might check all the aging electrical components as racing is a tough environment.

The car runs a Zork tube with the 02 fitting closer to the turbo exit. The AFR's are steady in the acceptable range when the car is running normally and it goes like a bat out of hell running exactly as it should. Then it falls, stumbles and the AFR falls off the map on the lean side and doesn't read anything. However i'm not so sure the AFR is really getting an accurate reading at that time with all the popping and sputtering out the exhaust.

The car has two Holley fuel pumps that pull fuel from the ATL fuel cell into a canister (the name of it escapes me at the moment) to avoid starving of fuel for hard cornering. I will post a picture below (top left of the first picture). A factory 930 pump then sends the fuel to the rear where it is picked up by the secondary pump at the dog leg. The pumps are hard wired into the car.

Originally Posted by mark houghton

Just to add to Brian's comments, if you do indeed have poor electrical connections - particularly to the WUR, that could cause you to develop a rich condition when the car is warm (no power to the WUR would cause it to revert back to cold start enrichment settings). The same could happen if you lost the rear fuel pump relay, since the power from that is what feeds the WUR.

Thank you for the input. This line of thinking makes perfect sense, however as I hadn't mentioned they are wired in directly when the power is on.

Is there any chance that the WUR itself fails? I took it apart and had a look at it, there was a loose washer inside that i'm assuming was used as a shim on top of the springs to change the flow rate? Anyway I removed the washer and cleaned the WUR out and re-installed it. Same result, car got hot after a few laps and then got into its fits of stumbling.

[Adam144]

Things to note:

- The car won't replicate the problem in the garage at home even after idling for extended periods of time and getting plenty warm.

- The stumbling isn't just when on boost. The car stumbles and runs rough even down at idle once the issue starts happening.

[RarlyL8]

Tough to diagnose a non-stock system.

One thought as it is non-stock, is it possible that you are experiencing vapor lock?
Temps at idle sitting are nothing like when on boost racing.

[Adam144]

Originally Posted by RarlyL8

Tough to diagnose a non-stock system.

One thought as it is non-stock, is it possible that you are experiencing vapor lock?
Temps at idle sitting are nothing like when on boost racing.

Very much so a possibility. I know the temps are far higher under load than at idle and even moreso under racing conditions.

Any suggestions on preventing/eliminating a vapor lock scenario?

[smurfbus]

Have you checked your distributor cap, rotor and the green wire? Rotor might be burned if high resistance.

[Adam144]

Originally Posted by smurfbus

Have you checked your distributor cap, rotor and the green wire? Rotor might be burned if high resistance.

All have been replaced after the problem appeared the first time.

[87 930]

Sounds like a classic ignition coil issue. I see that you're running the Blaster and it's well thought of. Where's it mounted? Next time the engine craps out on you, carefully cool the coil off with some cooling spray and go drive it again. An easy, quick check.

[Adam144]

Originally Posted by 87 930

Sounds like a classic ignition coil issue. I see that you're running the Blaster and it's well thought of. Where's it mounted? Next time the engine craps out on you, carefully cool the coil off with some cooling spray and go drive it again. An easy, quick check.

The problem persisted even after swapping in a brand new spare blaster coil.

However it is mounted on the fan shroud (we did get a bracket and intend to move it to the left side of the engine bay above the factory relay panel).

[87 930]

Adam; Gottcha, new coil. When I have a heat related issue, I focus on cooling components that could cause the problem. Now, I'd lean toward Brian's and Mark's ideas. Cool the relays one at a time on the engine electrical panel and drive it. Feel for hot wires under the front hood (esp fuel pumps and relays). I can't see how the stuff under the driver's seat (IE; speed relay) would get that hot, but I'd tap on it when the problem is occurring. Electrical connections can "open" when they're hot. Do a wiggle test on all conns when it's crappin' out.

[Adam144]

Is anyone well versed in how the warm up regulator and fuel distributor work that could better explain them to me? Or even any links to their functions? Haven't had much luck yet in my search.

[smurfbus]

Search for jfairmans posts. Lots of good info of fueling 101.

[gsxrken]

Originally Posted by Adam144

Is anyone well versed in how the warm up regulator and fuel distributor work that could better explain them to me? Or even any links to their functions? Haven't had much luck yet in my search.

The bible:

Amazon.com: Bosch Fuel Injection and Engine Management: How to Understand, Service and Modify (9780837603001): Charles O. Probst: Books

[mark houghton]

Originally Posted by gsxrken

The bible:

Amazon.com: Bosch Fuel Injection and Engine Management: How to Understand, Service and Modify (9780837603001): Charles O. Probst: Books

Yep, a must-have for anyone running CIS injection.

[Ronnie's.930]

Three helpfull threads to look at -

Advanced CIS Turbo Tuning Discussion Thread. Ultimate?

ELECTRONIC CIS fueling & BOOST control

CIS test gauge - procedure?