diff --git a/code/map_data_cache.js b/code/map_data_cache.js
index 2b221f41..c143e9c9 100644
--- a/code/map_data_cache.js
+++ b/code/map_data_cache.js
@@ -2,20 +2,19 @@
 // cache for map data tiles. 
 
 window.DataCache = function() {
-  this.REQUEST_CACHE_FRESH_AGE = 90;  // if younger than this, use data in the cache rather than fetching from the server
-  this.REQUEST_CACHE_MAX_AGE = 60*60;  // maximum cache age. entries are deleted from the cache after this time
+  this.REQUEST_CACHE_FRESH_AGE = 60;  // if younger than this, use data in the cache rather than fetching from the server
+  this.REQUEST_CACHE_MAX_AGE = 180;  // maximum cache age. entries are deleted from the cache after this time
 
   if (L.Browser.mobile) {
     // on mobile devices, smaller cache size
-    this.REQUEST_CACHE_MIN_SIZE = 200;  // if fewer than this many entries, don't expire any
-    this.REQUEST_CACHE_MAX_SIZE = 1000;  // if more than this many entries, expire early
+    this.REQUEST_CACHE_MAX_SIZE = 300;  // if more than this many entries, expire early
   } else {
     // but on desktop, allow more
-    this.REQUEST_CACHE_MIN_SIZE = 500;  // if fewer than this many entries, don't expire any
-    this.REQUEST_CACHE_MAX_SIZE = 4000;  // if more than this many entries, expire early
+    this.REQUEST_CACHE_MAX_SIZE = 1000;  // if more than this many entries, expire early
   }
 
   this._cache = {};
+  this._interval = undefined;
 
 }
 
@@ -51,16 +50,29 @@ window.DataCache.prototype.isFresh = function(qk) {
   return undefined;
 }
 
+window.DataCache.prototype.startExpireInterval = function(period) {
+  if (this._interval === undefined) {
+    var savedContext = this;
+    this._interval = setInterval (function() { savedContext.runExpire(); }, period*1000);
+  }
+}
 
-window.DataCache.prototype.expire = function() {
+window.DataCache.prototype.stopExpireInterval = function() {
+  if (this._interval !== undefined) {
+    stopInterval (this._interval);
+    this._interval = undefined;
+  }
+}
+
+
+
+window.DataCache.prototype.runExpire = function() {
   var d = new Date();
   var t = d.getTime()-this.REQUEST_CACHE_MAX_AGE*1000;
 
   var cacheSize = Object.keys(this._cache).length;
 
   for(var qk in this._cache) {
-    // stop processing once we hit the minimum size
-    if (cacheSize < this.REQUEST_CACHE_MIN_SIZE) break;
 
     // fixme? our MAX_SIZE test here assumes we're processing the oldest first. this relies
     // on looping over object properties in the order they were added. this is true in most browsers,