Boxxer 421 Heat issue

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New Member
Dec 28, 2010
Real Name
Business Location
United States
To start, has been a pleasure following the forums here for a few days now...

With that said. One of my vans which I have owned a while now, (few years, Boxxer 421 with less than 1200 hours on a NEW Briggs Engine... Original one crapped out.)

Unit has never been a primary clean unit for me. but other than constant, little issues and the need to replace engine it has run solid. 230-260 degree heat by end of pre-spray has never been an issue.

Month ago I took it to Interlink to get a service done on the unit, the only real problem they saw was that the throttle and thus RPM were apparently too high. (Had run this RPM since new or at least unadjusted. Whether by coincidence, or something shortly after dropping the throttle I began having issues getting to full heat.. Diverter valve arm throttling, light engaging, etc, but very hard time getting up to temperature and remaining there. Was now taking 10 minutes or more to break the 200 degree mark, sometimes getting to 230 or so, but then randomly dropping down rapidly under normal cleaning use. (Same wand, same tips.) Even as much as to drop all temperature showing on gauge by completion of a 30 minute job. (Very random acting.)

Then decide to take it to Jon Don, they do a full service, etc. I mention the throttle adjust which he inspects and claims is now a few hundred RPM below norm, and that this could be my issue... He adjusts the throttle back, runs the wand for a while and unit remains at heat. Of course assuming the unit is fixed I drive 2 hours home and prepare for my big night of cleaning under proper heat again.......

Same issues as described..

Have noticed when we seem to be having the most heat issues, I am getting surplus steam blasting from exhaust which I have never noticed before......?

Currently, Interlink nor Jon Don have any clue as to what the problem may be...

Suggestions please? Before I drive it 2 hours in either direction to Jon Don or Interlink again.
Feb 22, 2011
Real Name
Clint Morrill
Business Location
United States
I would check your vacuum level at full throttle it should read 14"hg, also make sure that the divertor is switching properly. Temperature control being out of calibration can also have it diverting too soon


Randy Dockins
Premium VIP
Nov 12, 2008
Real Name
Randy Dockins
Business Location
United States
You've probably already read this, but maybe this can help. Here is some advice from the Boxxer 421 manual. Almost certainly one of these things is causing your problem. You should easily be able to rule some of them out one at a time and hone in on what is the problem. My guess is you have a heat exchanger leak which will require service anyway. Your engine should be set at 3000 rpm at full throttle. A little more than that won't hurt a thing, but much under that could cause performance issues. I do not think that the extra steam out of the exhaust is normal from what you are saying and I would really make sure you have no leaks in either of your heat exchangers. I think this may be your whole problem. It may just be something needs to be tightened, but unless you have a forklift handy you won't be able to do it yourself.

2.0. Unable to achieve normal cleaning solution temperature
2.1. System is in Divert mode or temperature control knob is turned down. Inspect
divert switch and temperature control. Change or adjust.
2.2. Diverter valve is not closing completely. Inspect diverter valve for proper
operation. Adjust linkage as necessary.
2.3. Diverter and hot water dump systems will not switch into "Cleaning" mode.
Refer to qualified service technician to test system.
2.4. Engine RPM is too low. Check RPM with accurate tachometer and adjust as
necessary to 3000 RPM.
(Any small engine shop can do this if you can't or just install a Tiny Tach.)
2.5. Cleaning solution flow is too great. Measure flow at cleaning tool.
2.5.1. Cleaning tool jet is too large or worn out. Inspect jet. Replace if necessary.
2.5.2. Cleaning solution pressure is too high. Adjust pressure to normal. Inspect
pressure gauge for accurate reading.
2.6. Heat exchangers have hard water scale build up internally. Descale system.

1.0. Water from exhaust
1.1. The recovery tank has been filled with foam or overfilled with water. Remove
recovery tank lid and inspect. If full, drain tank then inspect high-level shutoff switch for
proper operation. Clean or replace switch as necessary.
1.1.1. If foam is observed in recovery tank, use defoamer on carpet being cleaned.
1.2. Condensation. This will be more pronounced in cool weather and humid climates.
Observe how long this condition persists after starting machine. If it is only until the
machine warms up, it is normal.
1.3. A heat exchanger is leaking. Change the diverter mode switch back and forth
between Divert and Heat Exchange mode. Observe which condition causes water to be
expelled from exhaust. This would be my guess is your problem.
1.3.1. If water is expelled while switch is in Heat Exchange mode, the engine exhaust
afterburner heat exchanger is leaking internally. Remove and test. Replace as

1.3.2. If water is expelled while switch is in Divert mode, the blower exhaust heat
exchanger is leaking. Remove and test. Replace as necessary.
You may want to call Hydramaster technical support about this.