Heat-Soak Issue... a solution.


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Jan 6, 2015
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If you've found your way to this thread, your venerable 6th Gen Accord is probably experiencing the gremlin known as "heat soak"--which, in turn, causes a "hard start condition"--and/or its more insidious companion, the dreaded "random misfire" (P030x) DTC code.

First some backstory into this difficult-to-troubleshoot-and-therefore-difficult-to-resolve problem.

Plenty of anecdotal, and often contradictory, opinion is available among the various Honda forums and on YouTube that propose the cause of both, along with their purported solutions. Both are known issues with Honda proper, with TSB's relating to both. But Honda doesn't have a firm grasp of its cause either.

The consensus regarding the cause of the "heat soak" condition seems to be, ambient heat from a hot engine compartment radiates into the fuel rails when the engine has run at normal operating temperature, then shut off. The temperature of the pressurized fuel increases to a non-specific point where the ethanol additive vaporizes within the fuel rails. The result is the engine will not start; or, if it starts the idle is intermittent and the engine stalls--a phenomenon Honda terms "hard start condition"; the ECU lights the MIL and reports a "random misfire" P0300 code, often accompanied by one or more P030x cylinder-specific misfire codes.

The consensus regarding the causes of the misfire codes is violently less clear. No one--from the grizzled Honda mechanic to the savvy Honda engineer--can draw a definitive connection that explains why--or even IF--a connection actually exists! One can reasonably argue that a "hard start condition" is simply another condition among a very long list of issues that causes the ECU to report "misfire" codes, which was a generic catch-all code for that generation engine/ECU combination. Experience dictates that a hard start condition will always produce a misfire code; however, the appearance of a misfire code does not indicate the presence of a hard start condition. If your Accord suffers from intermittent-but-chronic occurrence of misfire codes, attempt to isolate its cause using the following list, in no particular order:

- carbon build-up
- fuel injector(s) issue;
- glitchy ECU/PCM;
- blown radiator fan fuse (this one makes zero sense to me. ...Honda Techs?);
- air trapped in coolant system;
- bad... coolant temperature sensor; or, O2 sensor(s); or, crank shaft position sensor;
- overly tight I/E valves;
- coil issue;
- EGR issue;
- bad plugs and/or ignition coils;
- main fuel relay solder joints.

Yeah, yeah... So what about potential solution to Heat Soak

The primary symptom of "Heat soak", the "hard start condition", is not subtle. Fortunately, the list of possible solutions to the problem is far shorter.

- generally poor-quality fuel--switch brands (octane rating is irrelevant to this problem);
- ethanol fuel vaporization in fuel line--try zero-ethanol fuel
- fuel pressure regulator too small (Honda TSB 00-024)--Honda requires replacing the ECU when replace the FPR;
- returnless fuel rails--replace with fuel rails from 2000 Odyssey.

I opted to replace the fuel rails. I confess to feeling a smidgen intimidated at the notion of re-plumbing the fuel system of my Accord. But after I was knuckles-deep into the job, I found it was pretty straightforward and just not that big of a deal. Consider yourself reassured. Now let's get to it...

First... Read a Mechanic's Experience with the Heat Soak Condition
To gain some appreciation for the technical rationale for yanking out the Accord's native fuel rails, please begin by reading the following remarkable two-part Motor Magazine article published in June/July 2015. The mechanic took an engineer's empirical approach to troubleshoot this ornery problem, and in doing so invented the procedure you will do on your own Accord, so read it. Part 1 and Part 2.

You may notice the article does not describe the procedure for swapping the fuel rails. In the interest of brevity (believe it or not) I do NOT write the procedure for doing this job either. Instead, throughout I defer to YouTube for procedures. What I provide henceforth is a series of advisories based on my job notes and experience upon the successful execution of this job. That said...

Doing the Deed

- the full and contiguous "top-end" fuel system from a 2000 Odyssey.
Why a 2000 Odyssey? Because everything fits exactly where it is supposed to with zero modification necessary. You will remove the Accord's "top-end" fuel system and you will bolt on Odyssey's fuel system. I purchased my parts used from Beaver Honda Salvage in North Carolina ($65 at the time of this post). You can certainly use new parts, if that appeals to your sensibilities or because you cannot source used parts, though expect an exponential increase in cost, of course.
- 24" new fuel hose
- 12" new vacuum hose
- fuel injector o-rings/seals kit (Option: Replace existing injectors set of remanufactured OEM fuel injectors)
- four 12oz bottles of carb/throttle body cleaner, and a jug of engine degreaser (a.k.a., "Purple Power")

2000 Accord "returnless" fuel rails. Notice the rail ends are in fact plugged. Pressurized fuel sits in the rails and cannot return to the tank.


2000 Accord "returnless" fuel rails. The FPR and the damper are mounted to the same block

2000 Odyssey fuel rails. Notice the configuration differences.

2000 Odyssey fuel rails. Fuel circulates through the rails then back to the tank, staying relatively cool

ADVISORY 1: Preparing the Parts
When the Odyssey parts arrive, inspect that everything is present by comparing to the images above. Clear-up any missing or confusing part issues now.
- If you purchased used parts, thoroughly clean the whole shebang before installing. Replace the old vacuum and fuel hoses from the FPR with the entire length of new.
- If you purchased new parts, assemble the whole shebang as one contiguous string of parts. Attach the full length of new vacuum and fuel hoses to the FPR.
Remove the fuel pulsation damper from the Odyssey's damper block, then remove and discard the old fuel feed line (the hose with the banjo connector). Remove the Accord's fuel feed line (a hose that runs between its connection at the firewall to the damper) and secure it to the Odyssey's damper. Don't forget to replace the metal compression ring!

The job requires you to remove the intake manifold and the entire top-end fuel system from its feed/return connections at the firewall. Plenty of good YouTube videos detail the process of removing these parts from a 2000 Honda Accord v6, so study any relevant videos carefully.

ADVISORY 2: Fuel Injectors
Carefully remove each fuel injector from the Accord fuel rails. Plenty of good YouTube videos detail the process of removing/reinstalling fuel injectors in a 2000 Honda Accord v6.
- If you're reusing old fuel injectors:
Clean each fuel injector (Plenty of good YouTube videos demonstrate how.)
Replace old o-rings with new. Do NOT fit the injector seat (thick o-ring) to the bottom of the injector. Set it aside (see ADVISORY 3).
Carefully press each injector into the fuel rail.
- If you purchased new/refurbished fuel injectors:
Carefully remove the injector seat (thick o-ring on the base of the injector) from the bottom of the injector. Set it aside (see ADVISORY 3).
Carefully press each injector into the fuel rail.
NOTE: The assembled fuel system will be unwieldy. The plastic pintel caps on the bottom of the fuel injectors CAN BREAK if they are bumped around. Handle the squeaky-clean fuel system carefully from this point forward.

With the necessary whatnots removed, take the opportunity to do some cleaning. Using generous amounts of carb cleaner and degreaser, clean the...
- engine surfaces and inspect every nook for oil seepage;
- intake manifold and intake manifold gasket, and remove and clean vacuum manifold and gasket;
- EGR Valve, the down-port in the injector base, and the EGR port in the water passage housing (the thing the EGR Valve bolts to);
- throttle body;
- intake manifold mating surfaces. Be certain to cover the intake holes throughout the job.

ADVISORY 3: Installing the Injector Seats
Carefully fit each injector seat down into the injector base--this eases installation of fuel injectors considerably. Follow the videos that demonstrate the procedures for re-installing fuel rails.

ADVISORY 4: Interference at the Front Fuel Rail
The rigid fuel pipe that connects to the right (return) side of the front fuel rail will NOT clear the corner of the injector base. No worries! Simply remove the 6mm fastener that secures the rigid fuel pipe to the fuel rail. With that thing removed, the fuel rail slips into position. After seating the injectors and securing the fuel rail to the injector base with the two fasteners, reattach the rigid fuel pipe to the fuel rail--consider replacing the original o-ring around the brass nipple with a new o-ring--and tighten the 6mm fastener well.

Top view of the interference. (Front of engine at image bottom) Yellow mark indicates fuel pipe connection mounted at the end of the fuel rail. This view clearly shows how the injector base blocks the rigid fuel pipe.
(See that black orifice? That's the EGR down-port. Clean it! ...but remove the EGR Valve beforehand, or you will regret it later.)

Side view of the interference. (Front of engine at image left) Yellow mark indicates fuel pipe connection.
Obviously sufficient room for the rigid fuel pipe: zero contact with the injector base when mounted. Happy times.

ADVISORY 5: Fuel Feed Line
Connect the fuel feed line (the hose dangling from the damper) to the rigid fuel line at the firewall and secure it in place with the existing hose clamp.

ADVISORY 6: Running the New Hoses
After installing the damper block to the two mounting studs:
- carefully feed the new vacuum hose along the path of the old hose to its connection at the intake manifold vacuum port--cut the hose only after it is in place with no pinches, interferences, or contact with metal surfaces.
- carefully feed the new return fuel hose along the path of the old return fuel hose to its connection to the rigid fuel line at the firewall--but do NOT connect it to the rigid fuel line yet. I strongly encourage securing the new fuel hose to nearby lines with zip-ties. Cut the hose only after it is in place with no pinches, interferences, or contact with metal surfaces.

After installing the Odyssey's fuel system but before installing the intake manifold, reconnect the battery and ready your assistant. An assistant will turn the ignition to Accessory ONLY, and ONLY when you ask.

You are going to push air out of the Odyssey's fuel system and replace it with fuel. You expect fuel to exit the end of the return fuel line with some force into a container that you will hold, so be prepared. One attempt should be all that is required; two possibly. If, after a third attempt, you don't see any fuel from that hose, you have a blockage in the system that you must find. The procedure is:
1) Grasp the end of the new return fuel hose and hold a container to catch fuel as it is pumped out;
2) Ask assistant to turn the ignition to Accessory ONLY. Close the driver door--you need to listen for the fuel pump relay click on and off. After two seconds, the relay will click "open" and the fuel pump will stop.
3) When fuel appears from the hose, press the fuel hose onto the rigid fuel line and secure it with the hose clamp. (If no fuel appears, or appears as a very small amount, repeat.)
4) With the fuel hose secured, continue to cycle the fuel pump five more times in 10 second intervals.
5) Inspect all fuel hose clamp connections and the banjo connection at the damper for leaks, and fix any that are discovered.
6) Remove the key from the ignition, disconnect the battery and proceed with the installation of the intake manifold.
I recommend you do not reinstall the plastic engine covers until after the test drive.

ADVISORY 7: Post-Test Drive
After you've driven the car for a few miles, park and pop the hood, then...
- Intake manifold bolts - check torque (16lbs);
- Carefully inspect all fuel hose clamp connections and the banjo connection at the damper for leaks;
- Sniff--yes sniff--at each fuel injector location through the intake manifold plenum for exhaust or fuel odors. (Note: I prefer visual specificity over olfactory. So I dumped an ounce of oil-safe florescent dye into the Accord's tank prior to the test drive, then scrutinized the length of the plumbing under a UV light. I found no leaks. This step is entirely optional, but it's prudent.)
If you discover an issue during your inspection, remedy it. Plenty of good YouTube videos detail each of the points of inspection mentioned above.

This job is a solution for the "hot soak" condition ONLY; and of consequence, you eliminate one cause for a misfire code ("hard start condition") should the ECU report one in the future. Also... though the "hot soak" condition does cause a very specific, very temporary idle problem, swapping the fuel rails does NOT fix idle problems. If you have an idle hunting issue, look up "IAC Valve troubleshooting" and proceed as instructed.
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Well-Known Member
Mar 24, 2010
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New York
Yep very nice write up. I had saw a different post maybe from a different site where someone also did that. Or was it this forum and IIRC i PMed the guy if it was a permanent fix and he never got back. I think his name is jester or something. Nevertheless I just decided to live with it for now.