306 Ranger, pulling timing at tip in

I am having an issue with my 306 V8 Ranger that seems to come and go. Any feedback or brain storming ideas would be appreciated as to what might be causing the issue. I will try to provide as much information as possible to help identify what might be occurring. I will include a list of components used in the build a little further down page. The package now has approx. 200,000 miles on the build. The problem comes and goes with long periods of time running without issue.

The truck is pulling spark timing at Tip in. Off the line under normal load/semi aggressive load it will stumble and shutter. We can see the spark curve through a datalog drop to 7-9 degrees when this occurs and then return back to normal. The truck is down slightly on power throughout the rpm range but normally runs fairly well. At times there is an intermittent issue with bucking and a very noticible mis-like condition through out the rpm range under load and through all gears. That issue comes and goes and seems to be brought on when the spark plugs are changed. It feels like bad gas but some times stays for several tanks of fuel or 1000 miles.

Here is what has been done to identify and correct the issue so far.

Changed spark plugs several times, at least 4 times. Motorcraft SPF432, Autolite 103,104,764. Experimenting with gap ranging from 44-54, mainly 54.

Changed plug wires. Stock Motorcraft Explorer replacement.

Replaced, MAF, fuel filter, TPS, Cam position sensor, ACT, EGR, DPMS, O2 sensors, coil packs.

Went to a back up ECM and a back up custom tune.

Had full induction system cleaning with BG treatment. Injectors showing good pulse and flow.

Smoke test, no vacuum leaks anywhere.

Compression test, all cylinders between 149-152.

Scoped the truck on three different devices. Nothing unusual.

Did have occasional codes, po401, po402 prior to EGR and throttle body being cleaned. Codes are now gone.

Sent a camera down each spark plug hole and down each intake runner. Extremely clean and tight everywhere. No unusual play in cam timing.

Here is what has not been done.

Crank positioning sensor, IAC (although it was cleaned). Cam synchronizer. PCV valve.

My main question, "what would cause it to pull timing at tip in?"

Build details
306 ci, stock block and deck height, .030 over bore
PATS disabled
Custom tune
Stock crank
Scat con rods
Silvolite Hyperutectic pistons, 9.5 comp, 1605 comp height.
Everything weight matched to plus or minus .5 grams.
Entire assembly balanced with balancer, flywheel, and clutch pack.
Comp cams Xe-264-hr cam
Ford Racing Timing set
Ford Racing lifters
Crane push rods
Stock 1.6 rockers
Ported GT40P heads, .004 milled, Trickflow springs.
Ported GT40 intake
Gasket matched throttle body elbow
24 lbs injectors
Stock Explorer throttle body
Walbro 195 fuel pump
TM Headers
Custom mandrel bent 2 1/2 exhaust through two Flowmaster 44's
T5 z-spec 5 speed trans
3.55 Trac Lok rear.
29 inch tire

Does it have a knock sensor?

adsm08 said:

Does it have a knock sensor?

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Not that I know of unless it goes by a technical description I am unfamiliar with. It is 100% 2001 Explorer except for a custom tune, 180 degree thermostat, some performance parts, 5 speed, rear O2 sensors delete, custom exhaust, and PATS disabled. It has all the stock sensors including the full EGR system. The coolant temp sensor that runs the dash gauge has also been replaced with a Autometer adapter and gauge. That sensor does not feed the ECM.

Spark timing still works the same as the old distributor and points setup days
RPMs dictate spark advance
The 14.7:1 fuel mix takes XXX amount of time to fully ignite after spark, that doesn't change, but the time it takes for piston to move from 10deg BTDC to 10deg ATDC changes with RPM.
So distributors had weights and springs that would advance the spark as RPMs increased, so full ignition would happen at 8-10deg ATDC so piston and connecting rod had good leverage to push down on the crank, before that and leverage is not as good and after that you don't get the full power added to the crank.

But there is a problem with just RPM spark advance
A richer fuel mix burns a little faster, so XX time from Spark to full ignition
And that's why Vacuum Advance was added, it retards the spark slightly as vacuum in the intake dropped because of the Richer fuel mix burning faster during acceleration, Knock sensor would delay or reverse this to ignite the mix before it could self ignite

Computer controlled spark follows the same "rules", it uses a base RPM advance and then retards it based on throttle position and air:fuel mix ratio, and should be far more accurate than the old system.
Computer calculates manifold pressure and barometric pressure, it doesn't have actual sensors for those, but both would be based on MAF, RPMs and TPS data

So if a symptom of the stumble is a change in spark advance, I would look at the tune and what parameter would cause that change in spark timing under acceleration.

Not a tuner, just general engine stuff

Almost sounds like you're losing fuel flow/pressure under load

RonD said:

Spark timing still works the same as the old distributor and points setup days
RPMs dictate spark advance
The 14.7:1 fuel mix takes XXX amount of time to fully ignite after spark, that doesn't change, but the time it takes for piston to move from 10deg BTDC to 10deg ATDC changes with RPM.
So distributors had weights and springs that would advance the spark as RPMs increased, so full ignition would happen at 8-10deg ATDC so piston and connecting rod had good leverage to push down on the crank, before that and leverage is not as good and after that you don't get the full power added to the crank.

But there is a problem with just RPM spark advance
A richer fuel mix burns a little faster, so XX time from Spark to full ignition
And that's why Vacuum Advance was added, it retards the spark slightly as vacuum in the intake dropped because of the Richer fuel mix burning faster during acceleration, Knock sensor would delay or reverse this to ignite the mix before it could self ignite

Computer controlled spark follows the same "rules", it uses a base RPM advance and then retards it based on throttle position and air:fuel mix ratio, and should be far more accurate than the old system.
Computer calculates manifold pressure and barometric pressure, it doesn't have actual sensors for those, but both would be based on MAF, RPMs and TPS data

So if a symptom of the stumble is a change in spark advance, I would look at the tune and what parameter would cause that change in spark timing under acceleration.

Not a tuner, just general engine stuff

Click to expand...

Thank you very much for the detailed response. We have considered that exact scenario as a possibllity. Back when this package was first assembled I was working with my tuner to overcome a similar issue at Tip in. I considered the possiblity that there was something built into the custom tune that could be contributing to the issue I am experiencing now. If that is the case I imagine I will need to seek out a new tuner to work with me via data logging to develop a new custom tune.

I can't be 100% sure yet without a little more seat time but it feels as if the Tip in issue does not exist while the truck is in Open Loop. At least by "seat of the pants" feel. I plan to pay a little more attention during cold start ups over the next few days and possibly do some more data logging next weekend where it can be reviewed while in Open Loop.

win said:

Almost sounds like you're losing fuel flow/pressure under load

Click to expand...

I thought about that as well Win. While it was in the shop Friday they checked fuel pressure and it was a pretty steady 55 through the RPM range.

I might mention that the ST Fuel Trim for Bank 1 seems to run just slightly lower then the ST Fuel Trim for Bank 2, not much and it does start to equalize through the RPM range.

I don't nothin' about the tune but something caught my attention recently re: GT-40P heads which I'm also using. The spark plugs are different length than the old E7 heads and at one point I wasn't sure I had the correct plugs in mine.

Just a thought
Computer sets barometric pressure at startup, this is for air weight calculations and height above sea level, so it can run engine in Denver as well as Death Valley, lol.
The bigger CAM will change that BARO number I believe.
Check what computer thinks the BARO is and then check local airport or weather station online for current barometric pressure.

The actual fuel mix would be compensated for by O2s if BARO was off, but not sure about spark because of Richer or Leaner mix

Do these have a vvt ?

Junglejoe said:

Do these have a vvt ?

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No, Variable Valve Timing is used on some Over Head Cam engines, the 306 is OHV, a pushrod engine, it is an over bored 302(5.0l) OHV
Not sure any one has done VVT on any OHV engine?

gm has....hemi.......



its the actual fuel...and your very keen sensitivity.


200 k and rolling dynamic compression.

RonD said:

Just a thought
Computer sets barometric pressure at startup, this is for air weight calculations and height above sea level, so it can run engine in Denver as well as Death Valley, lol.
The bigger CAM will change that BARO number I believe.
Check what computer thinks the BARO is and then check local airport or weather station online for current barometric pressure.

The actual fuel mix would be compensated for by O2s if BARO was off, but not sure about spark because of Richer or Leaner mix

Click to expand...

Thanks Ron,

I thought that might be a possibility as well. I did not keep good notes as to when this problem has occurred in the past and at what time of the year. Going from memory it seems to be around July, August, September. It is possible that the tune is just that sensitive and only causes issue at certain times of the year.

Not sure if I mentioned this before but the issue does not occur in open loop, or in closed loop immediately after a start up (within two or three seconds). It also does not occur if I roll to a stop and come off the pedals quickly thereafter (start moving) before the computer has a chance to catch the idle.

Jeff you ever figure out this strange issue?

Junglejoe said:

Jeff you ever figure out this strange issue?

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Sorry, I have had trouble with the profile page for awhile now. No, I never did figure it out. It seems to come in the hot months each year. It is gone now that the temp has changed in the cooler months. Just to be clear, the problem with timing at tip in is still present but no drivability issues continue. The drivability issue was a stumbling at tip and that is gone. It is possible that the two are connected but only causes a issue when ambient temps and humidity are up.

I am still working with my son on a new custom tune to work around the timing dropping at tip in. We purchased the SCT Software to edit the existing tune or create a new custom tune all together. While gearing up for that the truck has developed a new issue. Rocker noise above 2700 rpm's. I have adjusted the preload three times. I have replaced the lifters, rolled the pushrods on glass, and Inspected all components. I guess the next step is replacing the valve springs unless I can identify the problem in another area. It sure sounds like a rocker slapping.