I have a 88 ford ranger 2.9 when it is cold it starts to surge and the check enginge light comes on. The light will stay on and go off as the surging ramps up and down. It ranges from 800 to 2000 rpms. After the engine has run awhile it stops. I have replaced the o2 sensor, voltage regulator and did a tune up with no luck and suggestions would be great thanks.

 

What trouble codes do you get from the computer?

 

The only error code I get is 13

 

KOER 13 is the only trouble code? Does that mean you get pass (11) from both KOEO and continuous memory (CM)? When the CEL comes on while the engine's running it should store a code in CM. If it isn't storing a code, then that suggests to me that there might be something wrong in the wiring to the computer. Might check KAM power supply, main PCM power supply, and computer grounds.
Have you checked fuel pressure? vacuum? compression? base timing (SPOUT removed)? other?

 

Repulled the codes this morning and got a 51 98 and 13 code. What would be a good route to take next? Thanks

 

According to my code list 98 and 13 have to be KOER codes. 51 either KOEO, KOER, or CM. What type of code is the 51: KOEO, KOER, or CM?
I don't see 51's very often, so that makes me think that there is a short in the ECT sensor circuit somewhere. If this is a hard fault (KOEO/KOER code), then it shouldn't be too hard to track down with a voltmeter and a wiring diagram.