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Thread: Startup: Compensating for heat soak/off time.

  1. #1

    Default Startup: Compensating for heat soak/off time.

    Iíve never really seen an answer on this, so figured Iíd ask:

    What (if anything) can be done to adjust starting parameters to compensate for how long an engine has been shut off? The factory ECUs Iíve tuned all have an engine ďoff timeĒ table that's a multiplier for cranking fuel. The longer an engine is switched off, the more fuel is typically required.

    Iíve been trying to really dial in my startups, but itís just never consistent. Maybe my expectations are too high, but this seems to be why. After all, a 170įF start as the engine is warming up and has been parked a few minutes is quite different than a 170įF start after parking the car for an hour or two after full operating temp.

    Thoughts? Iíve read all the startup threads, and I donít have any real starting issues. Itís just sometimes very clean & crisp, and other times not so much ó usually depending on how long itís been parked warm/hot. Thanks

  2. #2
    Join Date
    Dec 2009


    I've never needed that time aspect. However, I did spend months tuning my Startup Enrichment tables.

    If you take the time to study this, you'll be a startup tuning expert, and read the "TIP" in post #6: (Good Starting Advice Thread - Especially posts #2, #4 & #6.)

  3. #3


    @BMWM.D.: You're overthinking this. If your cold start parameters are right and your MAT slope is dialed in, you should have clean starts every time and under all conditions. Andrew

  4. #4
    Join Date
    Dec 2009


    I categorically agree.
    May God's grace bless you in the Lord Jesus Christ.
    '92 Ford Mustang GT: 385" SBF, Dart SHP 8.2 block, TFS T/W 11R 205 heads, 232į-244į duration/.623" lift/114į LSA camshaft, 12:1 C/R, TFS R-Series FTI ported intake, BBK 80mm T/B, Dominator MPFI & DIS, 36-1 crank trigger/1x cam sync, 200A 3G alternator, Optima Red battery, A/C, 100HP progressive dry direct-port NOS, Spal dual 12" fans/3-core Frostbite aluminum radiator, Pypes dual 2.5" exhaust/off-road X-pipe/shorty headers, -6AN fuel system plumbing, Walbro 255 LPH pump, S&W subframe connectors, LenTech Strip Terminator wide-ratio AOD/3000 RPM converter, B&M Hammer shifter, FPP aluminum driveshaft, FPP 3.31 gears, Cobra Trac-Lok differential, Moser 31 spline axles, '04 Cobra 4-disc brakes, '93 Cobra booster & M/C, 5-lug Bullitt wheels & 245/45R17 M/T Street Comp tires.

  5. #5


    I didnít think it used MAT for cranking fuel control. Is this working behind the scenes or something?
    As another thought, I wonder if having my exhaust crossover open could be contributing to this. This is on a Pontiac 400 for what itís worth.

  6. #6


    Your correct BMWM.D., the Sniper does not use the MAT for cranking. But is your problem during cranking or after the Crank-To -Run number?

    I do believe in a majority of cases an engine off time table would be beneficial or even a fuel temperature table. Obviously that's not an option for now. So for now if you concentrate on what the engine wants in the vast majority of those scenarios, you'll find a happy medium with options available in the Startup Enrichment tables.

    I have a similar issue with one I'm testing and working with now. I believe it's partially a heat soak issue in regard to the intake manifold in conjunction to heat soak of the fuel. I think I now have to wait for ambient temperatures to rise again this spring to continue the testing. But my next step is to route a fuel line completely away from the heat of the exhaust for testing.

    Has anyone else noticed any unusual AFRs on startup when the fuel potentially could be just below the boiling threshold and MAT is being dictated by heat soak and don't seem to normalize until the MAT comes closer to IAT?
    Last edited by a b c; 11-01-2020 at 11:59 AM.

  7. #7


    Iíve been wondering if my exhaust crossover could be contributing to my issue. Itís a Pontiac 400 with Edelbrock heads, but the heads have the exhaust crossover ports and I donít have them blocked off.

    Iím still doing more testing, but mine seems to run lean after a heat soaked startup (CTS & MAT @ 150įF). The last time I checked it, it was running about 15% leaner vs when it was parked hot. Strangely, shutting the engine off and restarting seemed to bring it back to normal.

    That's a good theory about fuel temperature. I do have a factory fuel line that runs near the passenger side header. Maybe Iíll try some heat shielding.

  8. #8


    I still havenít been able to solve this with tuning. Letting the engine heat soak (MAT in the 150įF+ range) results in a lean start & idle. Adding After Start Enrichment fuel helps eliminate any driveability side effects (surging after it starts, etc.), but seems more like a band-aid fix than anything. I have what I believe to be a good MAT curve (thanks Andrew), and Closed Loop & Learn will be damn near zero when I park it. Immediate restarts are great.

    If youíre reading this A B C, youíve got a PM.

    My thoughts/plan at this point:
    1) My exhaust heat crossover is open. This canít be helping the situation. Exhaust gas runs directly under the plenum. Iím going to block it off.

    2) Phenolic spacer. Iím going to add a spacer to try to help insulate the Sniper from heat.

    3) Intake manifold: My Performer RPM intake has a full divider between the two sides. Iím not sure this is related to this particular issue I have, but after doing some reading it seems like not the best choice for the Sniper. This manifold is also tall enough to prevent my use of a phenolic spacer. So Iím going to swap to a single plane that should fit my packaging needs.

    As a side note, I did insulate my fuel feed line where it runs near my header. No difference there.

  9. #9


    BMW, what's the actual issue here? You say it runs "leaner on restart after heat soak", but doesn't Closed Loop adjust the fuel as needed? In other words, is there an actual driveability issue, or you just don't like what you see on the handheld? All of the data we get from the Sniper is great, but often times we get overly fixated on a small set of data points. If the car fires when you turn the key and idles OK, but you has a little more CL Comp, so be it, as long as it's not excessive.

    After Start Enrichment is only for a few seconds. How long does this lean condition occur? Have you posted a datalog of the event?

    I will say that the MAT is completely unreliable on the Sniper for tuning. It's not measuring the manifold air. It's measuring the temperature of the Sniper housing best I can tell. I say that because I can do a drag strip pass boosted and the MAT changes less than 3į. No way that is accurate. It should move 40-50į or maybe more as the boost builds heat.

  10. #10


    The main issue is that itíll actually stall on some warm restarts after it sits a few (3-4) hours. It'll fire instantly & cleanly, then goes dead lean and just shuts off like the key was turned off. It happens extremely quick, but I can see whatís going on if I datalog it. Then if I start it again, it fires right up and Closed Loop starts adding maybe 20%-25%. Sometimes I get some lean surges before Closed Loop gets it in check.

    Itíll also do the same thing without stalling if it sits heat soaked for less time, say 2 hours. Fires instantly and hits ~1100 RPM before doing the same type of clean stall behavior. But in this situation, it can recover before it stalls. I think I saw it dip to 350 RPM the last time I caught it on a datalog. This all happens very quickly.

    So yeah, itís a real problem Iíd like to get rid of. I wouldnít be too bothered by it if it werenít for the stalling in that particular situation.

    To answer your other question, the Closed Loop will reduce on its own after a while after these lean starts. I donít have any real data on that, but I think itís once the MAT starts dropping. You might be onto something with the MAT not being totally accurate when the Sniper gets really heat soaked. Maybe the air is actually cooler/denser than the MAT suggests. If so, I feel like my next steps of eliminating my exhaust crossover and adding a phenolic spacer make sense.

    In the meantime, adding extra After Start Enrichment fuel and/or Decay Time has helped. But then itíll run overly rich if I do a restart after only a short parking period in that particular coolant temperature range. So it definitely isnít ideal.

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